The effect of pyrimethamine and the combination of pyrimethamine-sulfadoxine (Fansidar) upon the termination of the acute attack of vivax malaria was studied in Thailand. Pyrimethamine was found to be ineffective, providing clearance of parasitaemia in only two of six patients by the end of seven days following treatment. The combination, administered in a two-tablet single dose (sulfadoxine 1 gm, pyrimethamine 50 mg) eliminated parasitaemia in only six of ten patients within seven days. Three tablets (sulfadoxine 1 . 5 gm, pyrimethamine 75 mg) given to 11 patients, provided clearance of parasitaemia in all within seven days; however, mean parasite and fever clearance times in this group were prolonged at 90 and 50 hours respectively. Chloroquine remains the drug of choice for the termination of the acute attack of vivax malaria. Subsequent primaquine is necessary for the prevention of relapse.