The clinical pharmacology of gentamicin was studied in 42 newborn infants. Gentamicin was found to be effective in vitro against most gram negative organisms encountered in neonatal infections; 1.5 mg/kg/dose every 8 to 12 hours in neonates produced mean peak serum levels of 2.5 to 5.0μg/ml. Serum antibacterial activity persisted for 8 to 12 hours. The serum gentamicin half-life was longer in premature than full-term infants during the first week of life only; beyond this time, the half-lives were similar and approached that reported in adults. No evidence of accumulation was observed after multiple doses. A limited trial of gentamicin in the treatment of severe neonatal infections demonstrated the drug to be effective. Acute toxicity was not observed. Specific recommendations and precautions are outlined for treating selected neonates with gentamicin.