An in vivo-in vitro model was used to assess the antimalarial activity of PS-15 and its metabolite, WR99210, against Plasmodium falciparum. WR99210, an antifolate triazine compound, was given as a single oral dose of 30 mg/kg to 8 Saimiri sciureus monkeys and, 3 months later, the parent compound, PS-15, was given similarly to the same monkeys. Serum samples were collected at various times after drug administration, serially diluted with control serum, and their antimalarial activity in vitro was determined against the multidrug-resistant K1 isolate of P. falciparum. Serum concentrations of PS-15 and WR99210 were estimated by high performance liquid chromatography. The maximum dilutions of serum that inhibited parasite growth were 20- to 86-fold higher 3 and 6 h after administration of PS-15 than following WR99210 administration. Substantial serum antimalarial activity was observed even at 48 h after medication with PS-15. Serum drug concentrations provided further evidence that PS-15 was absorbed far better from the gastrointestinal tract than WR99210. The substantial and sustained activity of PS-15 suggests that a single dose, or several smaller doses given once a day, should be effective in curing drug-resistant infections of P. falciparum.