Topical formulations of acyclovir (ACV) were tested in animals to define potential for tissue irritation and systemic toxicity. Acyclovir ointments (5 and 10% concentrations in polyethylene glycol vehicle) produced no sign of dermal irritation or systemic toxicity when applied to shaved abraded and intact skin of guinea pigs for 24 consecutive days. Solutions (0.9% normal saline vehicle) of ACV did not sensitize guinea pigs when 10 sensitizing doses and a challenge dose were injected intradermally. Petrolatum base ophthalmic ointments containing 1 and 3% ACV did not produce significant ocular irritation when applied to the corneas of New Zealand White rabbits 5 times each day for 21 consecutive days. A 6% petrolatum base ointment produced mild conjunctival irritation but no sign of corneal or iridic toxicity. Mean concentrations of 2.53 microM ACV were found in aqueous humor 2 hours after a 1 cm ribbon (21 mg) of 3% ophthalmic ointment was placed in the eyes of rabbits. A single treatment with a topical ointment containing 5% ACV in polyethylene glycol vehicle produced minimal irritation when placed in the eyes of New Zealand White rabbits.