The pharmacokinetic parameters of chlorproguanil (Lapudrine®) and its active metabolite, chlorcycloguanil, were determined in 6 healthy male volunteers after a single oral dose of 4 Lapudrine tablets (80 mg). The mean maximum plasma chlorproguanil concentration was 36.7 ± (SD)7.9 ng/ml and was reached at 3.8+1.3 h. The chlorproguanil elimination half-life was 17.5 ± 6.7 h and its plasma clearance was 1.28 ± 0.12 1/h/kg. The mean whole blood to plasma ratio was 3.1 at 4 h after dosing. Chlorcycloguanil could not be quantified in plasma and whole blood at the detection limit of 10 ng/ml using a high-performance liquid chromatographic method. An excretion rate-time plot from urine data shows a rapid (t½ = 20 h) and a slow phase (t½ = 51 h) in the elimination of chlorcycloguanil. Our findings suggest that the current prophylactic regimen of chlorproguanil hydrochloride (20 mg weekly) may not be optimal in preventing infections with chloroquine-resistant falciparum malaria.