CANADA IS ONE of the pleasantest places for an American president to visit. There is always friction between the two countries, but it is always manageable. When the president and the Canadian prime minister can agree, they usually have the satisfaction of having done something tangible for a lot of people with little of the danger that hangs over foreign policy in other parts of the world. Don't deride the relationship as boring; a boring serenity along a border, leaving countries' energies free for more productive preoccupations, is the reward of good diplomacy. The Quebec conversations this week between Mr. Reagan and Prime Minister Brian Mulroney were warm and at least modestly productive.

The agreement on acid rain is likely to prove more useul than it might look at first glance. A lot of Canadians have been needling Mr. Mulroney for having settled for a mere study. True, it's unfair that the wind blows smoke mostly from the United States into Canada and not vice versa. But this country has reduced emissions of sulfur dioxide, the most threatening of the pollutants, by one-third since the mid- 1970s. Canadians, meanwhile, have done very little to control their own prolific sources of acid rain. Canada would be in a stronger position to call for further action in this country if it began to tighten up the loose controls on its own power plants and -- especially -- smelters. Mr. Mulroney understands this point, and, with a little luck, the joint study will lead to a joint commitment to improve the protection of both countries' lakes, forests and, not incidentally, people.

The most sensitive subject in the Quebec conversations was defense, and Canada's fears -- exacerbated by a careless remark by Secretary of Defense Caspar Weinberger -- of being overrun by American strategic planners. But the more important subject was trade and money. Here the president and the prime minister did less than they might have done. Earlier the two countries were talking about a free- trade zone between them. But at Quebec, nobody seemed anxious to pursue such a radical thought. Instead, they proposed still another joint effort, this one to reduce tariffs and other trade barriers. This joint effort seems likely to produce less than it promises.

The reality is that, under Mr. Mulroney, Canada is again opening its economy to the foreign -- i.e., American -- investment that it needs. And under Mr. Reagan the United States is becoming more dependent on Canadian energy -- electricity in the East, natural gas in the West. These bonds of mutual advantage and dependence created the cordial atmosphere in which the two men met this week.