JOHANNESBURG, SEPT. 10 -- Nelson Mandela fever is sweeping South Africa again.

Rumors are rampant that the imprisoned 69-year-old leader of the outlawed African National Congress may be released soon. The speculation has fueled expectations of a breakthrough toward a negotiated settlement of the continuing political and social crisis.

Monday's dramatic, four-cornered prisoner exchange of 133 captured Angolan soldiers and two French and Dutch security prisoners for a white South African soldier captured two years ago in Angola has further heightened expectations of the release of Mandela, who last month completed his 25th year in prison.

The normally cautious Financial Mail, noting that nearly all of the conditions set for Mandela's release have been met, said today that the scene had been set for "the most dramatic political event in South Africa in nearly a quarter of a century."

The leader of the liberal opposition Progressive Federal Party, Colin Eglin, told Parliament yesterday that, with the completion of the prisoner swap, "the obvious question was whether {President Pieter W. Botha} was going to take any action in connection with the release of Mr. Mandela."

While there is no tangible evidence that Mandela's release is imminent, Botha and senior members of his government have dropped apparently intentional hints that the conditions for the parole of the ANC leader and other prominent security prisoners have been significantly modified and that it is no longer out of the question they could be freed without first renouncing violence.

Much of the current optimism about Mandela's release can be traced to Botha's statement to Parliament in January 1986 suggesting that if South African Army Capt. Wynand du Toit were released from an Angolan jail and if Soviet dissidents Andrei Sakharov and Natan (formerly Anatoly) Shcharansky were freed, Mandela could be released on humanitarian grounds.

At the time, the remark sent South African and foreign journalists and cameramen running to Cape Town, where Mandela is serving a life sentence for treason, to stake out Pollsmoor Prison for the expected emergence of the country's leading black nationalist.

Hopes for an early release quickly faded and remained largely dormant until du Toit's release Monday. Shcharansky was allowed to emigrate to Israel in February 1986, and Sakharov was freed from internal exile in December and allowed to move to Moscow.

The speculation already was intense last month when Botha said the renunciation of violence by Mandela and the other security prisoners, "like any other single positive factor, could contribute to a good prognosis {for release}, but is not decisive in its own right."

The president publicly offered Mandela his freedom several times in the past three years, with the provision that Mandela renounce violence in the struggle against apartheid and seek change through the constitutional framework.

Because Mandela has consistently rejected the precondition, arguing that as a prisoner he cannot unilaterally negotiate such a major ANC concession, Botha's most recent comment was seen as a retreat from his position. Government spokesmen have refused to elaborate on the president's statement.

On Aug. 13, Botha again gave momentum to speculation about Mandela when he said he had asked Minister of Justice Kobie Coetsee to look into the possible release of ANC leader Govan Mbeki from Cape Town's Robben Island Prison. Mbeki, 76, who was sentenced on treason charges with Mandela, has been in failing health and reportedly is nearly blind.

The speculative scenario is that after Mbeki is freed, the government will assess the reaction of South Africa's black majority and then consider releasing another prominent ANC leader, Walter Sisulu, 75, former ANC secretary general, who also was convicted of treason and sentenced to life imprisonment.

The government's aim, it is said, would be to keep to a minimum the emotional impact of an abrupt release of prominent ANC leaders.

Then, according to this scenario, Mandela would be released after a period of time and negotiations for power-sharing could begin.

Senior members of the government, apparently speaking with the approval of the president but on the condition that they not be identified, said the terms of Mandela's release could be included in the "talks about talks" that the government is attempting to initiate with what it considers credible black leaders.

A leading member of the parliamentary opposition, the Progressive Federal Party's Helen Suzman, said today that she is cautiously optimistic about the release of Mbeki as a test by the government. "He would be a lightning rod, so to speak, to test the reaction in the {black} townships and in the right wing and the white electorate," she said.

Analysts of black politics in South Africa said the most encouraging factor in Mandela's favor is that Botha's government increasingly sees itself as a political hostage to Mandela, who has become a symbol of the black struggle against apartheid. The realization is growing in Pretoria that even moderates who cooperate with the government will not participate in negotiations until Mandela is freed, according to the analysts.

It is also widely accepted that the government's fears that Mandela's release will trigger a jubilant resurgence in black revolutionary expectations have been partially offset by even greater fears that his death in prison might unleash more unrest than the country has ever seen.