The pharmacokinetics of primaquine have been studied in 13 G6PD normal and 13 G6PD deficient Thai male patients with Plasmodium vivax malaria who were given daily doses of 15 mg of primaquine over 14 d, following a full course of chloroquine. After the first dose (15 mg), primaquine underwent rapid absorption. Mean values (SD in parentheses) of maximum plasma concentration of 57.7 (7.7) vs. 55.7 (7.4) ng/mL were reached at 2.2 (0.6) vs. 2.2 (0.6) h, for the G6PD deficient and G6PD normal groups, respectively. Thereafter, drug levels declined rapidly and monoexponentially with a t1/2 lambda of 6.4 (1.9) vs. 6.3 (2.7) h. The respective mean values (SD in parentheses) for MRT, AUC0-varies; is directly proportional to Cl/f, and Vz/f were 6.8 (0.4) vs. 6.8 (0.5) h, 0.547 (0.070) vs. 0.521 (0.090) micrograms/h/mL, 8.54 (0.37) vs. 8.97 (1.46) mL/min/kg and 4.8 (1.7) vs. 5.1 (1.2) L/kg. There was no difference in the plasma concentrations or pharmacokinetics of primaquine between patients with normal G6PD and G6PD deficiency. In the G6PD deficient group, no relationship between the severity of haemolysis (< 20% or > 20% haemolysis) and the concentrations/pharmacokinetics of primaquine was observed.