The pharmacokinetics of primaquine (PQ) and its major carboxylic acid metabolite (PQC) have been studied in seven Indian patients with P. vivax malaria following PQ 15 mg/day p.o. for 14 days. After a single oral dose on Day 1, a mean peak blood concentration of 50.7 ng/ml PQ was attained after 2.3 h, which declined monoexponentially with a half-life of 5.6 h. The mean total body clearance was 37.6 l/h and the volume of distribution was 292 l. The mean renal excretion (0-24 h) of the drug was only 0.54% of the dose and renal clearance was 0.189 l/h. Following chronic administration, none of the pharmacokinetic parameters was affected, and a steady state blood concentration of 2.5-4.2 ng/ml PQ was attained. After the first dose of PQ, PQC had a mean area under the blood concentration - time curve 11-fold higher than that of the parent drug. In contrast to the rapid distribution and elimination of PQ, the metabolite showed a longer mean residence time and accumulation in the body. The mean Cmax and AUC of the metabolite on Day 14 were 48 and 40% higher than the corresponding Day 1 values. The metabolite could not be detected in urine at any time in any patient. PQ and its metabolite did not show any accumulation in blood cells.