Serum and urine concentrations were determined following the intramuscular administration of 50 mg/M2tobramycin and gentamicin sulfate to ten patients. The highest mean serum concentration was obtained at 30 minutes after the injections and was 4.6. mg/ml for tobramycin and 5.2 mg/ml for gentamicin sulfate. The serum half-life was 2.1 hours for tobramycin and 2.3 hours for gentamicin sulfate. About 60 per cent of the administered dose of tobramycin was excreted in the urine within six hours. Tobramycin also was administered at a dose of 50 mg/m2intravenously every six hours to seven patients. The highest serum concentration on day 7 was 6.0 mg/ml. There was evidence of some accumulation of drug during multiple dosage studies.