In order to investigate the efficacy of iontophoresis for increasing the penetration of vidarabine monophosphate into the eye, tritium-labeled vidarabine monophosphate was applied to rabbit eyes by topical and iontophoretic application, and the penetration of the compound into the eye, and its subsequent metabolism, were studied. At 20 min after treatment, the ratios of radioactivity for cathodal iontophoresis compared to topical application alone were cornea 8.6, aqueous humor 4.8, and iris 2.4; for 60 min the ratios were cornea 12.2 aqueous humor 17.5 and iris 2.5. In addition, the acid-soluble components were extracted from the cornea and aqueous humor. Vidarabine monophosphate, vidarabine, hypoxanthine arabinoside, adenosine, hypoxanthine, and adenine from the acid-soluble fraction were separated by thin-layer chromatography. The amount of vidarabine monophosphate and vidarabine in the cornea and aqueous humor from the iontophoretically treated group was six to 15 times higher than from the group that received topical application of the drug. It was concluded that cathodal iontophoresis resulted in significantly increased penetration of the antiviral drug vidarabine monophosphate into the anterior chamber of the eye. The effects of iontophoresis of vidarabine monophosphate on corneal epithelium, as observed by scanning electron micrographs, were equal to or less than those seen with the topical application of widely used preservatives in ophthalmic preparations.