Current local treatments of cytomegalovirus retinopathy may result in serious intraocular complications. Using an animal model, we investigated transscleral iontophoresis as a technique for delivery of foscarnet to the vitreous. Using a probe tip surface area of 0.19 mm2, a current of 1 mA, and a duration of ten minutes, transscleral iontophoresis of 0.5 ml of a 24-mg/ml foscarnet solution was administered to 72 normal rabbits. Vitreous aspiration was performed at 12 intervals (15 minutes, 30 minutes, and one, two, four, eight, 16, 24, 32, 40, 48, and 60 hours) after iontophoresis, and samples were analyzed by high-performance liquid chromatography to determine the vitreous pharmacokinetics of foscarnet. A peak foscarnet concentration of 200 +/- 31 microM (mean +/- standard deviation) was attained four hours after iontophoresis and was well below the concentration reported to cause retinal toxicity. Therapeutic levels were maintained until 60 hours after iontophoresis. The elimination half-life was approximately 24 hours. No toxic effects to anterior chamber structures were observed by biomicroscopy. Transscleral iontophoresis of foscarnet may provide an effective and safe technique for local treatment of cytomegalovirus retinopathy in patients with acquired immunodeficiency syndrome.