WHY SHOULDN'T Mexican goods flow north without the interference of tariffs and quotas, or American products flow south the same way? A free trade agreement would bring important benefits to both countries, and the model will be the United States-Canada agreement that went into effect two years ago. But Canada and the United States are countries with similar economies and similar standards of living. Opening up this country's southern border to free trade is going to be a much more intricate proposition.

Work is already getting underway. One of the purposes of President Bush's meeting this week with Carlos Salinas de Gortari, the president of Mexico, was to give a highly visible blessing to the enterprise. Canada is interested in joining the conversation, opening the possibility of a North American free trade zone reaching from the tropics to the Arctic.

As it now seems to be taking shape, it would have several limitations. To the regret of many Mexicans, it would not affect the restrictions on immigration and the movement of people. To the regret of many Americans, it would not change the prohibitions on foreign investment in Mexican oil fields or, like the Canadian agreement, establish access to energy across borders. But it could do a lot to create a single market for goods as well as for many kinds of services and investment.

One particularly tricky question will be the handling of environmental rules. Manufacturers in this country are now held to antipollution requirements that often drive up their costs. They should not have to compete with producers operating under much less stringent laws. But Mexicans argue that they can't afford to impose equal standards. That won't be resolved quickly. One promising possibility is, in time, to bring Mexican producers up to American levels of cleanliness -- with help from this country. A solution here could provide a highly useful precedent for environmental relations between the rich countries and the not-so-rich in the next century.

The talks with Mexico are getting started just as the great worldwide trade negotiations, the Uruguay Round, are coming to a climax. The purpose is the same in both -- more efficient markets, higher productivity, higher incomes. Trade agreements always face furious opposition from the industries that can't meet foreign competition. But the vehemence of that opposition is a pretty accurate indicator of the costs to consumers and to the country of trade protection at the border.