We have studied blood pressure and heart rate responses to standing in 29 previously ambulant adult Thai patients with acute uncomplicated falciparum malaria before and after treatment with quinine or mefloquine.There was significant, symptomatic, and usually profound orthostatic hypotension in 12 patients (41 %) before antimalarial treatment. The median maximum fall in systolic pressure was 24 mm Hg, significantly greater than the maximum fall in diastolic pressure 16 mm Hg.Blood pressure fell in two phases: an initial transient and usually asymptomatic fall immediately on standing, and a progressive, usually symptomatic fall, worsening over several minutes without a rise in pulse rate. Orthostatic hypotension was associated with core temperature (r = 0.37, P = 0.05).Antimalarial treatment accentuated the delayed orthostatic hypotension during malaria, despite (in the case of quinine) a significant reduction in fever. Both antimalarial drugs attenuated the cardioacceleratory response to symptomatic postural hypotension; the mean reduction in heart rate at the time of lowest blood pressure was 22 beats.min-1.The electrocardiograph ratio of RR intervals at the 30 th and 15 th beats was reduced significantly in acute malaria, but was not affected further by the drugs. When restudied in convalescence all the patients had normal postural cardiovascular responses.Acute falciparum malaria is associated with impaired circulatory control and the tendency to postural hypotension is worsened significantly by antimalarial treatment with the quinoline antimalarials quinine and mefloquine.