Three possible mechanisms of resistance to sulfadoxine were investigated in resistant Plasmodium falciparum: drug uptake, metabolism and alternate pathways. Uptake of [35S] sulfadoxine was markedly reduced in resistant plasmodia. By Thin Layer Radiochromatography it could be demonstrated that plasmodia do not metabolize sulfadoxine to pharmacologically inactive forms. Metabolism of sulfadoxine to the toxic analog of dihydropteroate is reduced in resistant plasmodia. Para-aminobenzoic acid (pABA) is not an essential nutrient for sulfonamide-resistant plasmodia. Instead, they seem to be able to synthesize pABA de novo. Four enzymes of the respective biosynthetic chain were demonstrated in isolated plasmodia: 3-deoxy-D-arabino-heptulosonate-7-phosphate synthetase (EC 126.96.36.199), shikimate dehydrogenase (EC 188.8.131.52), shikimate kinase (EC 184.108.40.206) and pABA synthetase. We conclude that these three effects account for the reduced sulfonamide stress observed in the resistant parasite.