Eighteen children with urinary tract infection were treated with intravenous drip infusion of gentamicin (GM), and clinical efficacy and pharmacokinetics were studied. Ages of the patients ranged from 2 months to 12 years. Doses of GM ranged 1.0 to 2.5 mg/kg every 8 to 12 hours, and treatment continued for 4 to 10 days. Among 18 patients treated, clinical results were excellent in 12, and good in 6. Values of BUN and creatinine remained within normal range in all patients during and after the GM treatment. One child had an eosinophilia. There were no cases that showed signs and symptoms of oto- and nephrotoxicity. Twenty eight time-serum level curves were studied in 16 patients during and after intravenous infusion of GM over 30 minutes. Doses were 1.0 mg/kg in 4, 1.9-2.0 mg/kg in 14, and 2.2-2.5 mg/kg in 10. Peak serum levels at 30 minutes after the start of infusion were 2.66-7.38 micrograms/ml (average 5.45 micrograms/ml) in cases receiving 1.0 mg/kg, 4.67-10.8 micrograms/ml (7.26 micrograms/ml) in 1.9-2.0 mg/kg, and 6.16-16.5 micrograms/ml (8.86 micrograms/ml) in 2.2-2.5 mg/kg. Elimination half-lives were 1.75-2.48 hours (average 2.10 hours) in cases with ages less than 1 year, 1.58-2.58 hours (2.01 hours) with 1 to 6 years, and 1.20-3.07 hours (1.66 hours) with 7 to 12 years who were given doses of 1.9-2.5 mg/kg. There were no significant differences in pharmacokinetic parameters between first and last administration in these patients, suggesting that no accumulation occurred with above mentioned doses. Urinary recovery of GM ranged from 21.9 to 99.2 percent (average 62.48%) within 6.5 hours after the initiation of drip infusion.