Twenty-five newborn infants with serious infections were treated with gentamicin. Blood and urine were collected to determine the patterns of absorption and excretion. Effective concentrations, with mean peak levels of approximately 3 µg/ml of serum, were achieved on a dosage schedule of 3 mg/kg every 12 hr. On doses of 2 mg/kg every 12 hr, the levels of gentamicin in serum were inadequate for therapy. Activity equivalent to approximately one-third of the administered dose was recovered in the urine. Infants treated with gentamicin included four with bacteremia, four with meningitis, two with urinary-tract infections, one with an infected wound, and 14 with pneumonia or clinical sepsis. The response to therapy was considered satisfactory in 21, indeterminate in three, and a failure in one newborn with meningitis due to Escherichia coli. No untoward effects of gentamicin were observed. On the basis of these studies, a dosage schedule for gentamicin of 6 mg/kg per day in two doses appears warranted for treatment of severe infections in the newborn.