There is a cool wind blowing south from Canada, and U.S. business is feeling the chill.

The breeze is spawned by the new wave of economic nationalism sweeping Canada, which has always been touchy about foreign investments: One-third of the nation's nonfinancial economy is foreign controlled and in some sectors, such as oil, the percentage is sharply higher.

Unlike previous surges of nationalism, such as the one about a decade ago, this campaign is being orchestrated by the government itself. Its main target now is the multinational oil company, but even long-standing foreign subsidiairies may feel some heat soon.

Prime Minister Pierre Elliot Trudeau and his Liberal Party were swept back into office in early 1980 on a platform that promised increased Canadian control of the economy (especially in energy) and a new constitution.

Indeed, some cynics have suggested that Trudeau's economic nationalism may be designed as much to create support for the new constitution as anything else. Most of Canada's 10 provinces, which have substantially more autonomy than U.S. states, are opposed to the new constitution, which would sharply increase the role of the federal government in Canadian life. Provincial challenges to Trudeau's attempt to "repatriate" the constitution are before Canada's Supreme Court now.

Furthermore, Trudeau is in a heated battle over oil and gas prices with the Western, oil-producing provinces. Alberta is so incensed over the prices the federal government permits for internal Canadian consumption that the province has reduced shipments to the East by 120,000 barrels a day and plans another 60,000 barrel cutback come Sept. 1.

"Trudeau discovered economic nationalism was good politics," said a major U.S. banker who follows Canada carefully.

Whatever Trudeau's motives, he has already proposed a new energy policy designed to reduce foreign ownership of the oil and gas industry from 72 percent to 50 percent by 1990 and to make Canada self-sufficient in energy. He has also promised tighter controls on non-energy subsidiairies of foreign companies operating in Canada, although those proposals are far from agreed upon among his cabinet members.

Trudeau's new energy program uses a set of discriminatory taxes and incentives to boost Canadian ownership of oil and gas companies. The program has already produced cries of foul play from a number of U.S. oil companies -- including Conoco Inc., which was maneuvered into giving up its Canadian assets by Dome Petroleum, and by Cities Service Inc., which is fighting a similar move by Ny-West Group Ltd.

Although Canadian officials deny it, there is growing concern in the United States that the new "Canadianization" move in the energy field, a seemingly tighter interpretation of permissible new foreign investment in Canada and the promised new investment rules are directed chiefly at U.S. companies.

"We're not discriminating against American companies," Energy Minister Marc Lalonde said in a recent interview. Both the energy program and investment guidelines apply to all foreign companies operating in Canada.

But even the most even-handed policies have to strike hardest at the United States. According to Canada's Foreign Investment Review Agency, nearly 80 percent of foreign investments in Canada are controlled by U.S. interests.

At the same time that Canada appears to be frostier to U.S. investors, Canadian companies appear to be getting more aggressive in the United States. In recent months, Seagram has tried to acquire St. Joe Minerals; the Belzberg brothers of Vancouver made a stab at The Bache Group, a big broker, and the Canadian Pacific tried to buy Hobart Corp. All these well-publicized takeover bids failed, in large part because of the Canadians seemed unprepared for the hard fight put up by U.S. managments.

But they served to drive home to the U.S. public the seeming divergence of tighter Canadian controls on U.S. investments while the U.S. remains wide open. Seagram, once thwarted, last week made a bid for control of Conoco Inc., now stripped of its Canada assets.

"It doesn't seem fair," said the chairman of a major U.S. company. "The Canadians can come down here and do anything they want, while Canada judges which U.S. investments it wants or doesn't want."

In 1974, the previous Trudeau government established the Foreign Investment Review Agency to screen all new Canadian investments (including takeovers) to determine whether they were of "significant benefit" to Canada. In its early years, the agency was mainly an irritant.FIRA head Gorse Howarth said in an interview that 91 percent of all applications were approved. Recently, he conceded, there has been a "marginal" increase in disallowances.

"That's pure hogwash," said a U.S. observer of Canada. "Those figures don't count in the companies dissuaded from investing because of the agency or those who went in for a chat and then decided not to apply." Furthermore, he said, FIRA has been getting testier (technically it recommends a course of action on each potential investment to the cabinet, which makes the decision -- all of it in secret).

In the first quarter of this year, only 75 percent of the applications were "allowed." The rest either were rejected or withdrawn.

Dow Jones & Co., the publisher of The Wall Street Journal, just lost a FIRA case in the Canadian Supreme Court. It brought a major book publisher, Richard D. Irwin of Homewood, Ill., that had a small Canadian operation. FIRA would not permit Dow Jones to take possession of the subsidiary. Similarly, Amoco Oil Co. put the Canadian subsidiary of a recent acquisition on the auction block, after FIRA rejected its application.

Canadian officials say they are mystified by the U.S. reaction to its foreign investment program.

Energy Minister Lalonde said he knows of no other industrial country that would permit such heavy foreign ownership of its oil and gas industry. "If the shoe were on the other foot, [the United States] would have taken steps a heck of a long time ago," he maintained. Lalonde said he applauds the Dome acquisition of Conoco assets and laments that the Liberal government didn't take steps to Canadize its oil and gas industry 10 years ago.

Howarth, of FIRA, said he does not understand cries from the United States, and elsewhere, that the investment guidelines are unfair. "I don't know how to approach it like that unless there was some degree of equivalence. kThe fact of the matter is that 33 percent of our nonfinancial economy is foreign-owned and controlled. The comparable figure for the United States is 2 percent."

On the record, U.S. companies are reticent. But several admit privately that FIRA is trying to extract fairly big concessions in return for approval of investment applications. Companies report the imposition of requirements such as that a company's Canadian subsidiary bank only in Canada, purchase a specific percentage of its requirements in Canada, or move operations from the United States to Canada.

Charles Byron, director of policy and research for FIRA, said the whole idea behind foreign investment review was to ensure that a new foreign subsidiary brought something to Canada.

Herbert Gray, Trudeau's minister of industry, is perhaps the country's leading exponent of economic nationalism and a severe critic of most foreign investments in Canada. In a report 10 years ago, Gray dubbed Canada the "branch-plant economy." To a large measure Gray is correct. Much of the foreign industry in Canada was set up to manufacture only for the Canadian market, admittedly inefficient in many cases. But foreign companies established the subsidiaries mainly because Canada erected high tariff barriers more than a century ago.

Gray, who proposed guidelines to the cabinet a year ago aimed at existing subsidiaries in Canada, wants to force increased research and development in Canada, more production for export rather than just the Canadian economy (global product mandates), and a review of the performance of all big foreign subsidiaries.

Lorne Lodge, a Canadian who is chairman of IBM Canada, said he believes that some "greater challenges" to foreign subsidiaries will emerge from the Trudeau administration.He said Trudeau administration. He said Trudeau has made too much of a commitment to a new investment policy to fail to act at some point. But he said he thinks the new measures will be less onerous than some fear.

He noted that it has been a year since Gray made his proposals. "Very little has happened since then," he added. "Indeed, it would appear that there is considerable disagreement within the cabinet."

IBM operations in Canada would be likely to meet Gray's approval. Its manufacturing operations produce two lines of computers for all IBM's North American operations and conduct extensive research and development for those computers.

Energy Minister Lalonde said that the cabinet has been concerned with energy and the constitution, but that the new proposals could be out late this year.

Some Washington officials are worried that Trudeau might inadvertently trigger a confrontation with the U.S. So far the Carter and Reagan administrations, while voicing some official concern about parts of the new energy policy, have not protested seriously any Canadian actions.

"But if the situation gets out of control of the executive branch," one official said, "It could escalate." Already there have been congressional hearings and last week a bill was introduced into the House to curb attempts by Canadian firms to strip U.S. oil companies of their Canadian assets.

Although the Reagan administration so far has publicly ignored what many in the United States feel to be a discriminatory Canadian policy toward oil investments, it might be forced to act under several statutes if a target U.S. company went to court.

James Gray, executive vice president of Canada Hunter Exploration Ltd. of Alberta has a similar worry. "I fully expect this confrontational government [of Trudeau's] before long to be angrily confronting the United States." In the end, he said in a recent speech, Canada has more to lose.

Canada will need hundreds of billions of dollars to develop its energy resources, much of which will have to come from foreign companies, despite the Canadianization desires of the Trudeau government. Furthermore, about 70 percent of Canada's exports go to the United States.

While international rules would prohibit discriminatory practices against Canada, an aroused U.S. public or business community could hurt Canadian exports. Canada is the United States' biggest export market too. Trade between the two countries hit about $75 billion last year.

Lalonde scoffs at multinational investments in Canadian oil. Since 1977, he said, foreign oil companies were net disinvestors in Canada, taking out $3 billion from the country. All oil and gas investment in Canada has been financed from within, he said.

One of the first steps Trudeau's government took under the new policy was a $1.46 billion acquisition of the Belgian oil company Petrofina's operations in Canada. The acquisition has been criticized widely. Petro-Canaca, the government's oil company, acquired a vast service station operation in eastern Canada, but dit not add a drop of oil to Canada's reserves, the critics charge. To pay for the acquisition, Canadian consumers are being taxed at 8 cents per [Imperial] gallon, merely to put maple leaves on gas pumps, critics said.

The acquisition moved Petro-Can from the 12th to the fifth biggest oil company in Canada.

Lalonde admits that the Petrofina acquisition added no new production to Canada. However, he said, Canadian companies have been more aggressive than non-Canadians in investing profits inexploration. Furthermore, he said, Petro-Can is eligible for greater incentive grants than Petrofina would have been, so the company will have more cash flow to reinvest.

Lalonde said that the hand-wringing in the United States is needless. Much of what has been written in the United States "is very misleading and distorted," he contends. "Some of the stuff verges on the ridiculous . . . Canada remains among the top countries in which to invest. That view is felt by Japanese investors and European investors . . . We have huge resources, a relatively small population and a damned good political record."

He almost laughs when U.S. concerns about Canadian takeover bids are mentioned. "We've been living with them [foreign companies] for 70 years." he said. "If the U.S. wants to set up a FIRA, it's up to them."

Despite this seemingly jaunty approach to foreign investment, few observers expect Trudeau or his ministers knowingly to take steps that would seriously alienate the United States or its foreign investors.

"U.S.-bashing is goods politics," said an American who is a long-time resident of Ottawa. "But the Canadian public will not tolerate a serious break with the U.S."

Furthermore, noted IBM's Lodge, although there is a certain amount of economic nationalism among Canadians and always has been, there has been no significant grass roots shift of opinion.

"If you ask the average citizen if there should be less foreign control of the Canadian economy, the answer will be yes. But in no way is it a high-priority item, "Lodge said.

Nevertheless, the likelihood is that until Trudeau gets his new constitution and irons out some of the bigger problems with the provinces, foreign investment will remain a favored political rallying point.

If he should miscalculate and allow the United States to perceive that its companies are being mistreated -- whatever the objective merits of the Canadian case -- the wind-chill factor in U.S.-Canadian relations could dip lower.