Opponents of Canada-U.S. free trade who do not wish to seem protectionist often suggest the special nature of the participants makes such a deal uniquely undesirable.

Free trade might be all right for some, they argue, but never have two such similar societies, with such unequal populations, been so closely linked. The spectre of cultural and economic domination is raised, along with ''massive'' adjustment costs.

In fact, the world has seen several bilateral free trade areas established in recent years, not a few of which have paired countries with similar languages and cultures, one of whom was overwhelmingly the larger.

While none of these is quite so sweeping as the Canada-U.S. agreement, in no case have the disastrous consequences forecast to stem from free trade come to pass, nor have any such treaties been revoked. In each case, the smaller partner has, if anything, been the biggest gainer.

The next step, some trade experts predict, will be to link these trade blocs into still larger free trade areas. Indeed, we may be seeing a major shift in strategy in the drive for world free trade, a task originally entrusted to the 93-nation General Agreement on Tariffs & Trade (GATT) in the heyday of postwar multilateralism.

''The GATT method gave us enormous gains for 20 years. Now the steam seems to be running out of it,'' says economist Richard Lipsey of the C. D. Howe Institute. ''Maybe this is the only route left.''

Instead of global gradualism, international trade liberalization may come about by means of these regional ''leaps of faith'': - New Zealand-Australia. These two countries mirror the Canadian-U.S. relationship in many ways. At less than three million, New Zealand's population is dwarfed by Australia's 15 million, and New Zealand has always felt insecure beside its richer, more dynamic antipodean mate.

They are, however, less dependent on each other's trade. In 1983, the year the Closer Economic Relationship (CER) came into effect, New Zealand accounted for just 6% of Australian exports, while Australia took 20% of New Zealand's exports.

The CER, as a classic free trade area, deals with goods only, not services and investment. Tariffs, higher in both countries than here, are to be removed faster - by 1988.

The last of each country's export incentives were scrapped this year, and each is to end preferences in government purchases. Import quotas were to be phased out by 1995, but this is likely to be accelerated.

''They put their toe in the water first, and then said 'let's jump in,' '' says New Zealand-born Tim Hazledine, an agricultural economist at University of British Columbia.

On the other hand, both countries retained their own anti-dumping and countervail laws, without recourse to a binational tribunal. Special exceptions were also made for some powerful interests, such as agriculture.

As early as 1985, a study by the New Zealand Institute of Economic Research and the Committee for Economic Development of Australia found a net gain in jobs for New Zealand. Emigration to Australia, 31,000 in 1981, has since halved.

Manufacturing investment in New Zealand surged, thanks in part to a devalued currency. Exports to Australia in the newly liberalized sectors grew at a 21% average annual rate. Where once there was a 4:1 imbalance in trade in New Zealand's direction, it is now in rough balance. - Ireland-Britain. Britain's population is 56 million, 15 times Ireland's 3.6 million. They share a common language: though Irish is the first official language of Ireland, English is the language of business. At the time of the 1965 Anglo-Irish Free Trade Agreement, about 70% of Ireland's exports went to Britain, and British investment was crucial to its economy. Like New Zealand, Ireland only embraced free trade after it was clear the country's highly protected economy was stagnating. Growth had fallen to 1% per year in the 1950s, and emigration had reduced the population to less than three million.

PROTECTED INDUSTRIES

Under the deal, tariff and nontariff barriers were to be removed over 10 years, though agriculture again remained protected. At the same time, Ireland opened its doors to investors.

The result: economic growth averaged close to 6% a year in the 1960s. While some protected industries, like jute, clothing, footwear and furniture, suffered, others, such as pharmaceuticals, computers, and medical equipment, have grown rapidly. The population decline has reversed.

In 1973, both countries joined the European Community, superseding the agreement. - Europe. The 12-member European Community is much more than a free trade area. The express intent when the Community was founded in 1957 was the political unification of the continent.

Not only is there free movement of goods, services, labor, and investment internally, behind a common external tariff, but there is an effort to harmonize regulations across the board. A supranational European Commission referees disputes and sets guidelines for the removal of discriminatory practices. Most EC states are in the European Monetary System of fixed exchange rates.

While the EC is a multilateral arrangement, it is interesting to note the situation of France and Belgium: a shared border, shared language, and huge population imbalance - 55 million to 10 million. Belgium has not disappeared to date.

More interesting still are the series of bilateral free trade treaties negotiated between the EC, population 300 million, and individual members of the European Free Trade Association in the early 1970s, after Britain pulled out of the EFTA to join the Community.

These agreements, involving Austria, Switzerland, Norway, Sweden, and Finland, are broadly similar to the EFTA arrangement itself. Almost all tariff and most nontariff barriers on goods were removed. The usual exemptions were made for agricultural quotas, nor were services and investment included. Dispute provisions were weaker than the Canada-U.S. arrangement, involving nonbinding joint commissions.

According to Murray Smith, director of international economics at the Institute for Research on Public Policy in Ottawa, a study of economic performance in the difficult years of 1973 to 1983 shows the EFTA countries outperformed the EC in productivity and real income growth.

Adjustment in every case turned out to be ''almost painless,'' he says. Large gains were registered in two-way intra-industry trade, with specialization occuring within industries, rather than whole sectoral shifts.

Austria, with a population of 7.6 million, neighboring eight-times-larger West Germany, had special reason to be frightened. Its industry, like Canada's, was dominated by German branch plants, established to avoid the high tariff wall between the two countries. In just five years, from 1973 to 1978, the tariff was removed. Yet rationalization as often as not favored Austrian production. - U.S.-Israel; U.S.-Mexico. The 1985 U.S.-Israel pact, the first bilateral deal involving the U.S., eliminates all tariffs within 10 years, on four different reduction schedules. While a joint commission will monitor trade disputes, the U.S. retains full right to impose countervailing duties.

With total trade between the two countries barely $5 billion, however, it is unlikely Israeli exports would ever cause ''significant harm'' to a U.S. industry, as countervail law requires.

Just days after the groundbreaking Canada-U.S. deal was signed, the U.S. announced it was about to sign an agreement with Mexico opening negotiations toward removing all tariffs and many nontariff barriers between the two countries by 2000.

It is contemplated that investment and services will be included. But the disputes settlement mechanism envisioned is purely consultative.

Mexico is the fourth-largest market for U.S. exports, while the U.S. is Mexico's largest export market. The deal comes just one year after the debt-ridden nation joined the GATT.