The kinetics of mefloquine was investigated following oral divided-doses in 10 healthy Caucasian volunteers. They received 500 or 750 mg followed by 500 mg 8 h later. Unchanged mefloquine (M) and its carboxylic acid metabolite (MM) were measured in whole blood and plasma for 50 days by h.p.l.c. Maximum blood and plasma M concentrations of 1872 +/- 362 ng ml-1 (mean +/- s.d.) and 1900 +/- 434 ng ml-1, respectively, were found within 6-10 h after the second dose. The terminal plasma elimination half-life was 20.1 +/- 3.7 days (mean +/- s.d.) and the oral clearance was 22.3 +/- 6.7 ml h-1 kg-1 (mean +/- s.d.). Plasma concentrations of MM exceeded those of M by 2-3 fold within 2 days. The whole blood concentration of MM was lower than that in plasma but also exceeded the whole blood concentration of M.