CANADA'S VOTERS, in exasperation, pitched Pierre Trudeau out of the prime minister's office nine months ago. Now they have voted him back. The Canadian condition hasn't changed much since last spring. But how about Mr. Trudeau -- has he changed?

The two central issues are still much the same as they were at last May's election, which ended 11 years of government under Mr. Trudeau and his Liberal Party. First come economics and oil, with choices very similar to those in the United States. Inflation is not quite so bad in Canada as in this country, but unemployment there is worse. The short-lived Conservative cabinet, under Joe Clark, decided to raise the controlled price of oil and to put a tax on gasoline. That hastened Mr. Clark's departure. Mr. Trudeau knows well as Mr. Clark -- or, for that matter, as well as Jimmy Carter -- that underpriced oil does intolerable damage to an industrial economy. But Mr. Trudeau has not given much indication of the changes that he might now make in the positions that lost last May's election for him.

The other great Canadian issue is, of course, national unity. Mr. Trudeau represents, vigorously, the principle of strong federal union. The great anomaly of Canadian politics is Quebec, which this week again voted massively for Mr. Trudeau but has a provincial government dedicated to separatism.

In the last days of the campaign, Mr. Trudeau reverted to the themes of economic nationalism and stronger central control of the economy. Those ideas will make Americans uneasy, because in the past they have generally been accompanied by a sharpening of all the disputes over trade and investment. But the most serious implications of Canadian economic nationalism are internal. It mainly favors those parts of the country -- Ontario and Quebec -- that live by manufacturing. The west, with its dependence on oil and agriculture, strongly supports an open economy in which it can buy and sell without restraint on world markets. The west resents the oil-price controls imposed from Ottawa -- the same controls that are extremely popular in the east.

Canada runs some 3,600 miles from west to east, and most of its people live in a narrow strip along its southern edge. This shoestring demography accentuates all of the differences of language, economy and region. Canadians have now returned to office a man of notable brilliance and experience, but one who seemed last spring to have become inflexible and worn by too many years in power. For Canada everything now depends on whether Mr. Trudeau considers this vote a mandate to return to old positions, or an opportunity to look for new ideas and new allies.