The concentrations of chloroquine and desethylchloroquine in the blood of 10 healthy adult Swedish volunteers who had been taking 310 mg chloroquine base once a week for at least 8 months for malaria prophylaxis were measured. Samples of capillary whole blood from the volunteers were dried on filter-paper and the drug and its principal metabolite determined by a specific high-performance liquid chromatography (HPLC) method. The day after taking the drug, the mean concentration of chloroquine and desethylchloroquine in whole blood were 1305 nmol/l and 915 nmol/l, respectively, and immediately before the next weekly dose, 489 nmol/l and 384 nmol/l, respectively. These are considered to be greater than the minimum inhibitory concentrations for susceptible strains but less than the maximum tolerated concentrations. The dosage of chloroquine recommended roughly 40 years ago for regular long-term prophylaxis should therefore not be changed.