Nine infants with symptomatic infections caused by herpes simplex virus or cytomegalovirus were treated with acyclovir. At the onset of therapy, the infants ranged in weight from 880 to 4550 gm. Five were premature. Acyclovir was administered intravenously in a dosage of 5 to 15 mg/kg every eight hours for five to 10 days. The peak serum acyclovir levels ranged from 20 to 163 microM and the trough levels ranged from 1 to 129 microM. The variation in peak serum acyclovir levels in different infants receiving the same dosage on a weight basis was large but correlated with the expected renal maturity of the individual infant. Hematologic values improved during therapy. No renal toxicity was noted. All of the infants survived, including the five with herpes simplex infections.