An improved and validated method is presented for the determination of proguanil, cycloguanil, and 4-chlorophenylbiguanide in plasma, whole blood, and urine using solid-phase extraction (SPE) technique and reversed-phase high-performance liquid chromatography (HPLC). The HPLC method uses isocratic elution with acetonitrile:phosphate buffer 0.1 mol/l, pH 2.6 (21.5:78.5 vol/vol) at a flow rate of 1.0 ml/min for the separation. The recovery of proguanil and metabolites ranged from 82% to 104%. The limit of determination was 20 nmol/l for proguanil and its metabolites in plasma and approximately 50 nmol/l for proguanil and metabolites in whole blood. Different stationary phases for HPLC and SPE were tested and the best chromatographic separation from endogenous constituents and other antimalarial drugs was achieved with cyanopropyl stationary phases.