1 Twelve children with acute falciparum malaria were treated with 25 mg/kg chloroquine orally in three divided doses at 24 h intervals. 2 Concentrations of chloroquine and its metabolite, desethylchloroquine, were measured in plasma from the beginning of treatment for up to 7 days using a high pressure liquid chromatography (h.p.l.c.) technique. 3 Chloroquine was detectable in plasma within 30 min of giving the drug. Peak level was reached in 1-8 h after the first dose of 10 mg/kg and the peak concentrations ranged between 65 and 263 ng/ml. 4 Chloroquine concentration declined slowly in plasma after stopping drug administration so that the concentration at the seventh day was 37.5% of the concentration on the third day. The apparent half-life was 3-4 days. 5 Desethylchloroquine was detectable in plasma within 30 min of giving chloroquine and peak levels were reached in 2-12 h. Peak concentration after the first dose of chloroquine ranged between 9 and 62 ng/ml. 6 Desethylchloroquine was also slowly cleared from plasma and mean concentration at the end of 7 days was 49% of the mean concentration at the end of 3 days.