Abstract. Chlorproguanil and chlorcycloguanil concentrations in human plasma and urine were measured using a high performance liquid chromatographic method. The detection limit in plasma and urine was 5 ng/ml for chlorproguanil and 10 ng/ml for chlorcycloguanil. The elimination half-life of chlorproguanil (Lapudrine®) in 2 healthy volunteers, after a single oral dose of the drug, was 14·7 and 16·1 h. No chlorcycloguanil could be detected in plasma over 32 h after dosing.