Maclean's, which bills itself on its front cover as "Canada's Newsmagazine," is about to go from an every-other-week to a weekly basis in September, fulfilling a protective government policy that drove a Canadian edition of Time out of business in 1976.

Three years ago, the national government in Ottawa put through what is known as Bill C-58, which made it impractical for Canadian advertisers to buy space in Time Canada, or in the Reader's Digest, which had a profitable Canadian operation.

Time's Canadian edition had a special six-page report on Canada, which was widely read and reputed to contain some of the best reporting on Canadian events. But in a response to charges - like one made by Maclean's editor, Peter C. Newman - that Time and Reader's Digest were "methodically . . . destroying Canada's periodical press," the government came to the rescue.

The simple curative of C-58 was to bar Canadian advertisers from deducting as a business expense advertising in any but Canadian-owned periodicals, removing the special exemption that Time and Reader's Digest had inherited over the years. Time dropped its Canadian edition after C-58 went into effect.

The Reader's Digest met the problem with an involved corporate reorganization that transferred 75 percent of the ownership of its British and French editions to Canadian hands. The Canadian edition continues to flourish under the new arrangement.

Canadian Secretary of State John Roberts argues that Time could have made a similar adjustment - a claim that non-Time sources vigorously deny. "They wanted to get Time out of Canada," says a diplomat. In any event, Time dropped its Canadian section and fired 40 editorial employes. It does maintain printing facilities in Canada for magazines still sold here.

According to an unpublished government report, Time's Canadian net revenue dropped from $1,629,000 in 1974 to $1,296,000 in 1977, but is picking up some this year. Time still enjoys a reputation as a good vehicle for business and international advertising.

Newman does not pretend that Maclean's has supplanted Time as an editorial product. The current twice-a-month version is far from the American concept of a newsweekly. It sia sort of hybrid - with news summaries similar to those of Time or Newsweek, accompnied by longer features of the kind that identified with the old Macleans's, a general magazine comparable to Colliers or the Saturday Evening Post.

A recent issue had a long cover story on "The Singles Myth," which concluded that things aren't working out the way "liberated" singles had hoped. The same issue had a long Q-and-A interview with Claude Ryan, the new Liberal Party leader in Quebec, and a feature-length piece on Mexico's economic recovery.

Even when it goes to a weekly basis, with more news and fewer long features, Maclean's is not likely to have the news depth of Time or Newsweek. Its foreign coverage will still be severely limited. The editors hope to have a man in London early next year. They maintain one correspondent in Washington and a part-timer in Paris.

Nevertheless, under Newman's guidance and the beneficence of C-58, Maclean's is flexing its muscles, expanding its offices in a modest building here to more than double its 35-member editorial staff for the weekly operation. Officials said that circulation, which has not changed much from about 673,000 before C-58, is expected to adjust downward slightly to 625,000 when its goes weekly - and to a per issue price of $1 instead of 75 cents.

"But that will be 33 million copies a year, tremendous for Canada," says the scholarly Newman, one of Canada's most successful authors and editors, Time Canada had a sale of about 525,000 - now down to about 300,000 - and Newsweek, which never had a Canadian edition, sells only about 55,000 to 60,000 copies of the American edition.

Newman defends the C-58 concept as necessary to stimulate Canadian investment in the publishing business, creating a climate in which firms like Maclean's "were not afraid to put big money into magazines - before that, it was a totally marginal operation. Maclean's lost money for the last decade or more."

Newman said he knew when he came to Maclean's in 1971, that he would either have to close it up or turn the "last general magazine in the world" into a newsmagazine. With the help of C-58, he is on the verge of accomplishing that.

Roberts sees the need to protect the Canadian magazine business as only one part of a larger whole. In an interview in Ottawa. Roberts complained that school children's TV consists, for three out of four hours, viewing of American programs. Two-thirds of all the books sold in Canada are published by foreign firms, mostly American.

Roberts' latest cultural worry is that out of about $230 million paid annually to rent motion pictures that for Hollywood films is about $65 million, and for Canadian films is just $3 million.

"The endangered species in the Canadian market is the Canadian expression of our own experience, the Canadian creative content," Roberts says.

Roberts says there must be greater "Canadazation" of the whole communications business. He regards television channels in Vermont, New Hampshire and the state of Washington as "poaching" on the Canadian market.

If some of Prime Minister Pierre Elliott Trudeau's advisers are a bit queasy about Roberts' approach, there does not seem to be much argument with what he is doing, His most recent protective effort has been on behalf of Canadian authors, who have violently objected to "remaindered" editions of their works published in the United States undercutting the sales of their higher-priced Canadian published works.

Canadians - government officials and private businessmen alike - are not in the least defensive when it his suggested that they may be pursuing a narrow intellectual nationalism.

"The distribution system of the United States," says Secretary Roberts, "threatens to overwhelm the expression of Canadian creativity."