Safe use of aminoglycosides requires close monitoring of serum concentrations. Limited information coupled with marked changes in fluid compartments and renal function during the first week of life in premature neonates makes interpretation of peak and trough levels very difficult. This study was designed to measure serum netilmicin levels following a 2.5 mg/kg IV push infusion. Blood samples were taken on the 5th day of therapy 1 hour before and 1, 6, and 11 hours after a dose. Fifteen premature infants weighing 1000-1500 gm at birth and 20 others whose weight ranged from 1501-2750 gm comprised the study population. All premature infants were appropriate for gestational age (AGA) and of them, only two were severely asphyxiated. At the time of the study, 10 neonates were still on respirators. Serum and urine sodium and creatinine, BUN, and urinalysis were obtained in 28 of these infants. No evidence of renal dysfunction was found. All infants received 100 mg/kg IV ampicillin every 12 hours, but none were being treated with diuretics. Serum netilmicin levels were measured by an enzymatic immunoassay, peak and trough were calculated by extrapolating the first order decay curve. Peak levels ranged from 3.4 to 14 micrograms/ml (means 6.1 +/- 2.5 micrograms/ml SD) and 90% of them were above 4 micrograms/ml. Half of the small premature infants (1000-1500 gm birthweight) presented trough values above 3 micrograms/ml. Pharmacokinetic analysis of our data predicts that a 2.5 mg/kg loading dose followed by 2 mg/kg given every 12 hours will decrease by one-half the number of small prematures exceeding the considered "safe" trough level (greater than 3 micrograms/ml).