Animal models of cutaneous herpes simplex and herpetic encephalitis were used to demonstrate the antiviral potential of acyclovir. Of the various topical formulations of acyclovir tested against herpes simplex in guinea pigs, 5 percent acyclovir in modified aqueous cream was the most effective. Propylene glycol was included in this preparation to increase the aqueous solubility of acyclovir. Acyclovir and vidarabine were found to be equally effective against herpetic encephalitis in mice when administered either orally or subcutaneously at 100 mg/kg.