Subconjunctival injection of 100 mg of aztreonam in rabbits with Pseudomonas aeruginosa endophthalmitis produced transiently high antibiotic concentrations in most intraocular sites. Concentrations of approximately 2.5 micrograms/ml were achieved in the vitreous humor 15 min after injection and persisted for 6 h. Repeated intramuscular injections in dosages of 25 mg/kg every 4 h resulted in drug concentrations in the vitreous humor of 2.4 micrograms/ml in infected eyes. These levels might be adequate for the treatment of intraocular infections caused by highly susceptible organisms. Direct intravitreal injection of 100 micrograms of aztreonam in normal rabbit eyes produced an estimated peak concentration in the vitreous humor of 62 micrograms/ml, with a half-life of 7.5 h, which declined to 6 micrograms/ml by 48 h. The pharmacokinetic indices suggest that, like other beta-lactam antibiotics, aztreonam is eliminated from the vitreous humor by the retinal route. This novel antibiotic warrants further study of its efficacy and toxicity to determine whether it may be a useful alternative to the aminoglycosides for selected cases of ocular infection.