Pharmacokinetics of acyclovir in the term human pregnancy and neonate. Concern about neonatal herpes often leads to cesarean delivery of infants in women with a history of genital herpes. The antiviral drug acyclovir has been used effectively to suppress genital herpes simplex virus recurrences in nonpregnant adults. Its administration to pregnant women with recurrent genital herpes may reduce herpes simplex virus recurrences and thus may decrease the cesarean section rate among this population. To study the pharmacokinetics, safety, and patient tolerance of suppressive oral acyclovir, either 200 mg (n = 7) or 400 mg (n = 8) was administered orally every 8 hours to pregnant women with a history of recurrent herpes simplex virus, from 38 weeks' gestation until delivery. The mean +/- SD plasma levels for the 200 and 400 mg groups, respectively, were: first dose peak, 1.7 +/- 0.6 and 2.3 +/- 1.0 mumol/L; steady-state trough, 0.7 +/- 0.3 and 0.8 +/- 0.6 mumol/L; steady-state peak, 1.9 +/- 1.0 and 3.3 +/- 1.0 mumol/L. In late gestation maternal acyclovir pharmacokinetics were similar to those of nonpregnant adults from other studies. Acyclovir was concentrated in the amniotic fluid; however, there was no accumulation in the fetus (mean maternal/infant plasma ratio at delivery was 1.3). Acyclovir was well tolerated, and no toxicity was seen in the mothers or infants. The administration of acyclovir, 400 mg every 8 hours, appears appropriate for use in an efficacy and safety study regarding suppression of herpes simplex virus recurrences during the last weeks of pregnancy. 