A method is described for the determination of the new antimalarial agent, mefloquine, in plasma and urine. 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 +/- 2.7 1 h-1, and apparent volume of distribution was 35.7 +/- 30.7 l kg-1 (assuming 100% bioavailability). 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.