Pharmacokinetics of gentamicin in children after intravenous infusion over 60 minutes were compared with that after intramuscular injection. 1. Mean measured peak serum levels after intravenous infusion of 2.5 mg/kg and intramuscular injection of 2.0 mg/kg were 6.1 micrograms/ml at termination of infusion and 6.5 micrograms/ml at 30 or 60 minutes after injection, respectively. Older children showed higher serum levels. 2. There was no difference in serum half-life between both modes of administration. 3. The AUC after intravenous infusion was slightly larger than that after intramuscular injection. 4. It was suggested that the efficacy and safety of the treatment by intravenous infusion in children are comparable to that by the intramuscular injection, and optimum single dose is 1.5--2.5 mg/kg.