In a headstrong moment in the heat of a parliamentary debate, a senior South African Cabinet minister has committed his government to a critical special election test that could mean sudden death either for the nation's new rightist party or the government's own reform program.

Minister of Labor Stephanus P. Botha, the second most senior member of the Cabinet, agreed last week to stand for reelection just when Prime Minister Pieter W. Botha had decided this was not a good time for a general election.

The labor minister, who is no relation to the prime minister, became involved in an angry exchange of words with the leader of the extreme Conservative Party, Andries P. Treurnicht, who broke away from the ruling National Party a year ago in protest against the government's plans to revise the strict segregationist policy known as apartheid.

In a moment of bravura, the labor minister challenged Treurnicht: "You resign your seat and I'll resign mine." That would mean both having to seek reelection in special contests. It was like a challenge to a duel.

Legislators occasionally toss challenges like this into debates, but they seldom are taken seriously. If the reelection contests are likely to be at all close, neither is prepared to run the risk of losing, so it remains a debating taunt.

However, this time Treurnicht decided within a few days to accept the challenge, leaving Botha honor-bound to go through with what could prove an uncomfortably tight fight. The date of the election has not yet been announced, but it is expected to take place in late April or early May.

If Botha loses, it will be a serious embarrassment for the government, and party insiders say the prime minister is furious with his namesake for running such a risk.

The stakes are so high on both sides that the leading Afrikaans newspaper here, Die Burger, has likened the contest to a game of Russian roulette. One or the other is going to get killed politically, it says.

If Treurnicht loses, it will be sudden death for him and his party, which now has 18 parliamentary seats. His constituency, Waterberg, is considered the most conservative in the country and if he cannot win there, his party cannot win anywhere. As his own colleagues in Parliament here admit, they will be finished.

If Botha loses, some observers think it could bring the government's reform program to a halt. He is minister of labor, an area where the government has introduced some of its most significant reforms. The conservatives say they will use that in their campaign to make him a symbol of the reform policy.

The terrain of this dueling ground is not favorable to the government. Both constituencies are in the heart of the northern Transvaal farmland, where white Afrikaner conservatism is deepest.

The region is also suffering from the worst drought in 50 years, adding to the hardships of recession for the farmers.

The conservatism of Treurnicht's Waterberg constituency is something of a legend in Afrikaner folklore.

Botha's constituency, Soutpansberg, is next door and runs through to the border of black-ruled Zimbabwe, whose proximity has its own effect on local white attitudes.

There will be supporting contests to the main duel. Botha included a second conservative in his challenge, daring Thomas Langley, who holds a Pretoria seat, to be his opponent in Soutpansberg. Langley has accepted and will also resign next Monday. His vacated Pretoria seat is one the liberal Progressive Federal Party could snatch with the segregationist vote split between the government and the Conservatives.

Meanwhile a National Party provincial councillor died the same day the challenges were being made, requiring a fourth special election, in a western Transvaal gold mining constituency. With anger among the white miners at Botha's labor reforms, this is another seat the Conservatives could win.

With so much against it, the government has one big thing in its favor. The Conservative Party has failed in efforts to make an election pact with the even more extremist Herstigte Nasionale Party, which is threatening to run spoiler candidates in all the seats.

Unless a pact is made before polling day, this could split the right-wing vote and prevent the Conservatives from winning any seats.