South Africa's leading opposition newspaper, the Rand Daily Mail, has set the pace for investigative reporting and pungent criticism of the government's racial policies for over two decades. Twice, it lost courageous editors when its publishers felt they had gone too far.

Now it has happened again, raising fears in the white liberal community that the paper, which helped bring down a prime minister by exposing South Africa's worst political scandal in 1978, has been left considerably stifled as a critic of government policies.

On June 1, the board of South African Associated Newspapers Ltd., saying it wanted to "rationalize the use of resources," announced that Allister Sparks, 48, was relinquishing his job as the Mail's editor and leaving the company after 22 years.

Under Sparks' five-year stewardship as editor, the Mail had relentlessly focused on the injustices of apartheid as well as the ambiguities and shortcomings of the government's reformist moves. Its expose of a scandal involving misuse of public funds within the Department of Information brought the Mail international acclaim. In 1979 Sparks shared the Atlas World Press Review's "international editor of the year" award.

During last April's general election the paper's coverage of the campaign was widely credited with helping the official opposition party to increase its parliamentary seats by 8 to 26.

But in a country where the ruling white minority does not like to be made too uncomfortable and at a time when the government's reformist posture has narrowed the scope of acceptable criticism among whites, Sparks' political line was incompatible with profits. The paper began to lose white readership and, as a result, advertising revenue. Informed sources at the paper said 1980 losses, $2.7 million, were the highest ever, and those projected for this year are over $4 million.

"Nobody buys a newspaper for the ads [so] if the readers don't find the news content satisfying, stimulating and presented in a way which holds them, then they're not going to see our ads," said advertising executive Darryl Phillips commenting on why ads had been withdrawn from the Mail. "From what we've seen over the last five years, [the Mail's] coverage has been angled at scoring political points as opposed to reporting the news in as objective a way as possible," he said.

When the Mail raised its price from 15 to 25 cents in an effort to solve its financial problems, circulation dropped further, from 135,000 to 110,000.

"On the other hand, it would be naive or wishful thinking to suppose that the Mail would be flourishing in the marketplace if only its politics were different," wrote the paper's ombudsman James Mcclurg, noting that many of the 11 dailies in the Johannesburg area were also unprofitable.

Nevertheless, the board's first remedy was to change the person responsible for its political orientation. Sparks' replacement is Tertius Myburgh, 46-year-old editor of South African Associated Newspapers' Sunday Times who is now editor-in-chief of both papers. Ken Owen, a former associate editor of the Sunday Times, was put into Sparks' slot as day-to-day editor.

Both men are regarded as more conservative than Sparks. Under them the Sunday Times became a major moneymaker with a circulation of over 450,000. Its editorial comment is milder than the Mail's was and in its format, politics takes a back seat to sex, scandal and sports stories.

Sparks' removal comes at a time when the white press is under the threat of further governmental restrictions. An inquiry into the media is pending, and it is widely viewed as the prescursor for more legislative curbs on reporting. There is hardly any black press left to speak of because of bannings of newspapers and black journalists.

What most anguished government opponents in the departure of Sparks was that the Mail's publisher accomplished what the government would dearly have loved to do -- remove its prickliest thorn. "I imagine they are chortling about this," said one company insider of the government.

"They botched it," he said of the board's decision. "It was muddled thinking. They could only think of the financial losses . . . and didn't foresee the political consequences."

The board's action has put an uncomfortable spotlight on the commitment to a virile opposition press among the English-speaking business and mining interests behind South African Associated Newspapers. Particular attention centers on Anglo-American, the giant mining empire of Harry Oppenheimer, a man who likes to cultivate an international reputation as a critic of apartheid.

Anglo-American money funds a blind trust that bought 21 percent of the newspaper group in 1975 to preserve the Mail's political orientation after a right-wing group tied to the government tried to buy into the company. A close associate of Oppenheimer knew of Sparks' impending dismissal six days before it occurred so it is likely Oppenheimer also knew. But according to one person knowledgeable about the situation, Oppenheimer said he could not tell the board how to run its affairs, especially when the paper was losing money.

But attention is now focused more on the consequences of Sparks' removal than its causes. One of the Mail's best features has been its meticulous reporting of the black labor scene, covering strikes and union-management clashes as the black labor unions find their ground under the government's new labor dispensation. How this coverage fares under the new editors will be a sign of the paper's new direction.

Myburgh has said the Mail "is not about to be turned on its head. This newspaper will continue to forage beyond the front lines in the cause of civil liberties, racial justice and full political participation for all."

But reporter John Kane-Berman expressed the sentiments of many when he commented that in place of the old Mail there will be "an ably edited paper, but one that will be less provocative to the government and less willing to offend the sensibilities of the whites in this rapidly polarizing society."