A rapid, sensitive and selective method was developed for the simultaneous determination of pyrimethamine and mefloquine, two of the active ingredients of Fansimef, in human plasma. The procedure involved extraction of the compounds and the internal standard nitrazepam from basified plasma with dichloromethane and chromatography on a C18 column (microBondapak, 300 X 3.9 mm I.D.) with acetonitrile-phosphate buffer as the mobile phase and UV detection at 222 nm. The limit of quantification was 10 ng/ml for both substances, using a 1-ml plasma specimen. The mean inter-assay precision was 2.8% for pyrimethamine and 4.7% for mefloquine up to 800 ng/ml. The practicability of the method was demonstrated by the analysis of more than 1200 plasma samples from several pharmacokinetic studies involving single-dose administration of Fansimef to both patients and volunteers.