Mefloquine base, (12.5 mg/kg body weight), was administered as a single oral dose to 34 pregnant women with Plasmodium falciparum parasitaemia. They were followed up in vivo using the modified 28-day WHO extended field test. The sensitivity of P. falciparum isolates obtained from these women to mefloquine (MQ) was evaluated in vitro. All women were parasite negative by day 4 and remained aparasitaemic throughout the 28-day period of observation. Parasitological and clinical responses were well correlated in all the patients. Minimal side effects, after drug intake, were reported by these women, but they all resolved spontaneously. The determined Mean Parasite Clearance Time (MPCT) was 57.7 +/- 14 hours. Seventeen parasite isolates were cultured in vitro; 9 (53%) grew satisfactorily. Schizont growth inhibitions was obtained at mefloquine concentration of 32 pmol/well (6.4 pmol/mu L). The effective drug concentration that gave 99% parasite growth inhibition (EC99) was 25.6 pmol/well (5.1 pmol/mu L); which indicates high parasite susceptibility to the drug in vitro. However, low dose of MQ may be ineffective in clearing parasitaemia in areas with mefloquine resistant parasite strains.