The chemotherapeutic efficacy of acyclovir was evaluated in guinea pigs that were inoculated intravaginally with herpes simplex virus type 2 (HSV-2). Acyclovir was administered systemically (50 mg/kg of body weight per day ip) beginning three days after inoculation and continuing for 10–11 days. Treatment with acyclovir reduced the incidence of paralysis of the hind limbs, mortality, and the severity and duration of genital lesions but had little effect on the excretion of virus from the genital tract. Recovery of HSV-2 from neural tissues of infected animals during latent infection was less frequent in acyclovir-treated animals than in untreated ones. These data suggest that acyclovir may be a promising drug for altering the severity of clinical genital herpes in humans during acute infection and reducing the incidence of viral latency following primary infection.