Concentrations of mefloquine in plasma and breast milk were measured in 2 women following the administration of one Lariam tablet (250 mg mefloquine). The milk-to-plasma ratio of mefloquine based on the area under the plasma and milk concentration curves was 0.13 and 0.16. After a maternal mefloquine dose of 3.73 mg/kg and assuming a daily milk ingestion of 1 litre over a week, the maximum amount of drug ingested by an infant would be 0.14 mg/kg. During lactation the plasma clearance and apparent volume of distribution of mefloquine were about 50% less than the same parameters calculated after lactation had ceased.