Toxicity of gentamicin and its use in the treatment of experimentally induced Pseudomonas aeruginosa endophthalmitis were studied in the postoperative aphakic eye. The retinal toxicity of intravitreal gentamicin was different from toxicity in the intact eye and varied with the method of injection. Successful prophylaxis of induced postoperative infection was obtained with an intravitreal injection of 30 mu-g of gentamicin but not with sub-Tenon's administration of gentamicin. Treatment for progressive endophthalmitis was effective with high dose (200 mu-g) gentamicin alone, or a combination of low-dose (30 mu-g) gentamicin with topical and systemic gentamicin. Either sub-conjunctival and systemic gentamicin or low-dose gentamicin alone was ineffective once endophthalmitis was in progress. Although vitrectomy removed abscess products, it failed in this experiment to provide better results than the other modes of therapy.