The pharmacokinetics of chloroquine and its desethyl metabolite were studied in six patients with chronic renal failure and compared with a control group of twelve patients with normal renal function. Chloroquine was given as a single oral dose of 600 mg and blood levels monitored for 144 h. Plasma chloroquine concentrations were determined using an HPLC method. The plasma half-life of chloroquine was significantly higher in renal patients than in controls. This finding suggests that extra caution should be exercised when prescribing chloroquine for prolonged use in patients with renal insufficiency.