Rightwing whites objecting to the government's reformist direction threw two separate political meetings of South Africa's governing National Party into chaos last night, jeering and taunting two presiding Cabinet ministers who were stumping for their party's candidates in by-elections Wednesday.

The incidents were viewed here by political observers as an indication of the growing right-wing revolt within the party because of moves by Prime Minister Pieter W. Botha to begin removing racial discrimination in the economic and social spheres as part of a conciliatory new approach to race relations in this country.

Results of the four by-elections will be taken as a gauge of how much support the party is actually losing. Already National Party supporters have stayed away from the polls in two recent by-elections to register their discontent, and some have cast their votes for the ultraconservative Herstigte (Purified) National Party that believes in the supremacy of the white race and complete segregation.

Last night's tumultuous meetings come only two weeks after Botha resolutely faced and rebuffed subtle criticism of his policies from delegates at the party's annual congress in the populous Transvaal Province. Since he launched his reformist trend almost three months ago, Botha has indicated he will not backtrack to placate the right-wing opposition even if it means losing some votes.

When a delegate took the floor to complain about the removal of segregation signs at post office windows Botha angrily replied: "We are always prepared to allow black people into our kitchens and to prepare our food and to bring us coffee in restaurants, but the moment a black man appears next to us in a post office we say 'go away'. What kind of nonsense is that?"

Botha challenged the delegates to oust him as their leader if they did not like his reformist direction. They did not pick up the gauntlet.

Last week, three separate announcements caused further rumblings on the right. First, government officials said restaurants could now apply for a once-only permit to serve blacks instead of having to reapply each year.

Secondly, Labor Minister Fanie Botha -- one of those booed and hissed last night -- announced that the government was dropping its initial stand that migrant laborers, who make up one-third of the country's work force, would not be allowed to unionize. All black workers now may join black unions that can gain legal status with the government.

Thirdly, Prime Minister Botha astonished rightists and apartheid foes alike by declaring his government was open to suggestions on how to change the laws against interracial sex and marriage.

Botha did not indicate how he might change the laws, which are regarded as cornerstones of the government's segregationist policy. But the indication that this was at least a possibility was hailed as a significant step.