1 A method is described for the determination of the new antimalarial agent, mefloquine, in plasma and urine. 2 After oral administration of 750 mg mefloquine to six volunteers, absorption was apparently slow, with plasma mefloquine concentrations at 24 h (559 + 181 ng ml-1; mean + s.d.) higher than at 6 h (459 + 166 ng ml-1). The elimination half-life was 373 + 249 h, oral clearance was 5.09 + 2.71 h-1, and apparent volume of distribution was 35.7 ± 30.71 kg-1 (assuming 100% bioavailability). 3 Mefloquine (750 mg) had no significant effect on salivary kinetics of antipyrine or on the metabolic clearance of antipyrine to its three main metabolites, 3-hydroxymethylantipyrine, 4-hydroxyantipyrine and norantipyrine, when antipyrine was administered either 2 h or 2 weeks after dosing with mefloquine.