Samples were extracted with n-hexane and the organic layer was rejected. 10 microl aliquots of the aqueous layer were injected onto the column. Amodiaquine was used as the internal standard. The UV detector response was linear over the range 0-1000 ng/ml with a correlation coefficient of 0.9987; the detection limits with respect to chloroquine and desethylchloroquine were found to be 20 ng/ml and 10 ng/ml respectively. Within-day and between-day assay variation was generally < 5%. No interference from endogenous constituents was observed. The utility of the method was demonstrated by determining chloroquine and its major metabolite, desethylchloroquine in plasma samples from six healthy human volunteers following a single oral dose of 600 mg of chloroquine. The procedure is simple and requires small volumes of plasma.