NEGOTIATORS for the United States and Canada will sit down this week to begin drafting an agreement for free trade. It's a tremendous venture -- and risky, in political terms, with high stakes for both countries. Very little attention is devoted to it here, but in Canada the prospect of open trade touches the deepest and most sensitive questions of Canadian nationality and Canada's relations with its huge neighbor. This drive for a free-trade agreement has already gone far enough that, if it now collapses, it will be followed by a time of anger, suspicion and tension between two countries that collaborate to their great mutual profit in the largest flow of trade across any border in the world.

The talks have been going on for more than a year, and the agreement must go to Congress by Oct. 5. Sometime next winter Congress will then vote on it under fast-track rules that prohibit amendments. On the American side, most of the opposition so far has come from narrow interests that are unlikely to pose any fatal threat. The doubts are more serious in Canada.

One issue is the treatment of the subsidies that are stitched through both economies. Another is the procedure for reconciling disputes over unfair trading -- including the use of those subsidies. What happens when a subsidized product from one country has unsubsidized competitors in the other? Many Canadian businessmen accuse American companies of using this country's infinitely complex trade laws to harass and impede imports. One thing that the Canadians want out of a free-trade agreement is assurance that the harassment and endless litigation will cease. That's going to be difficult to work out. Canadians also want a guarantee that American investors won't be allowed to buy up their country. In particular, there are great fears for Canada's cultural industries -- publishing, film, popular music. The implications of free trade for the Canadians go well beyond the conventional commercial quarrels.

For that reason the final decision on this agreement has to be Canada's. Unfortunately, Prime Minister Brian Mulroney's authority has slipped badly over the past year, and as the debate among Canadians has become more vehement, their political leadership has grown more hesitant and equivocal. The free-trade agreement would strengthen both countries' economies, expanding markets and opportunities for each. But Americans are in a delicate position. They would be right to support wholeheartedly the idea of free trade -- but they've got to be careful not to seem to be pushing it onto a Canada that is still sharply divided and troubled by it