The pharmacokinetics of trimethoprim administered orally or intravenously were investigated in six infants aged 1.7 months to 1.1 years. In these infants trimethoprim had a mean half life of 4.6 hours; this was comparable with the values found in young and school age children (3.8 and 5.4 hours respectively) and about a quarter of the half life in newborns. The volume of distribution (1.5 l/kg) was smaller than in newborns but larger than in young or school age children (0.9 and 1.1 l/kg respectively). The plasma clearance in these infants (3-3 ml/min/kg) was slightly larger than in newborns or in either group of older children (2.9 and 2.4 ml/min/kg respectively). Thus the most dramatic changes in trimethoprim pharmacokinetics seem to occur during the first two months of life. A reduced daily dose of trimethoprim is necessary during the first two months only. An increased daily dose, by addition of a third dose each day, is recommended from two months.