This study was carried out to investigate the pharmacokinetic and pharmacodynamic interactions between two antimalarial drugs, mefloquine and quinine. A randomized, comparative, three-way crossover study was performed in seven healthy male Thais after the administration of three drug regimens on three occasions i.e., a single oral dose of quinine sulfate (600 mg), mefloquine (750 mg) alone, or the combination of mefloquine (750 mg) and quinine (600 mg given 24 h after mefloquine). QTc interval was significantly prolonged in subjects following the combination regimen (at 2.5, 3, 4, 6, 8, 12, 18, 24 h after the quinine dose) but no abnormal clinical signs or symptoms were found. There were no significant changes in vital signs or routine laboratory values in any of the subjects. The pharmacokinetics of mefloquine and quinine were influenced by the presence of the other drug. Greater blood schizonticidal activities were collected from the sera of subjects on the combination regimen than from the sera of subjects the quinine or mefloquine regimens. The minimum inhibitory concentrations (MICs) of the equivalent concentrations (Eqs) of quinine or mefloquine, which completely inhibited the growth of the K1 strain of Plasmodium falciparum in vitro (MICs of quinine Eq and mefloquine Eq) were significantly lower in the sera of subjects on the combination regimens, than in the sera of subjects on mefloquine or quinine alone [MICs of quinine Eq: 41.2 (21.25-73.5) vs. 135 (118-150) ng/ml; MICs of mefloquine Eq: 18.2 (17-19.2) vs. 25.2 (24.4-26.8) ng/ml].