Experiments in rabbits have shown that the novel technique of transscleral iontophoresis is a safe, effective, noninvasive way to produce high concentrations of antibiotics in the vitreous humor. The authors have now studied the effects of repeated transscleral iontophoresis in the eyes of cynomolgus monkeys. Six treatments with gentamicin sulfate, 1.5 mA for 10 min, were applied to both eyes of three monkeys over a 2-week period. The concentrations of gentamicin in the vitreous humor 24 hr after treatment were determined for each eye on three occasions. The mean concentration was 28 .mu.g/ml (range, 11-44 .mu.g/ml) after the first treatment and 12 .mu.g/ml (range, 2-21 .mu.g/ml) after the fifth treatment. Indirect ophthalmoscopy completed after the last treatment showed only small retinal burns up to 2.5 mm in diameter in four eyes in the area of the pars plana over which the electrode had been applied. Electroretinograms were normal after treatment. These experiments confirm the efficacy and safety of transscleral iontophoresis in the primate''s eye and suggest that investigations of this potentially useful technique are warranted in humans.