Genital herpes simplex virus type 2 (HSV-2) infections in mice and guinea pigs were used to determine the effectiveness of acyclovir administered topically, orally, or parenterally. Topical treatment with 1 percent or 5 percent acyclovir begun six or 24 hours and 5 percent acyclovir begun 48 or 72 hours after intravaginal inoculation of mice with HSV-2 significantly inhibited viral replication in the genital tract. Oral administration of acyclovir resulted in a significant reduction in vaginal virus titers when therapy was begun as late as 72 hours after infection. Topical treatment with 5 percent acyclovir initiated 24, 48, or 96 hours after intravaginal inoculation of guinea pigs with HSV-2 significantly altered lesion severity and virus titers but not vaginal HSV titers. Oral acyclovir therapy begun 24 or 48 hours after infection also significantly reduced the mean lesion score and prevented their development in some animals. Lesion and vaginal virus titers were only partially reduced. Intramuscular administration of acyclovir begun on day 2 or day 4 of infection significantly reduced the mean lesion scores but failed to alter lesion or vaginal virus titers.