We examined the effects of inflammation (S. aureus endophthalmitis) and of probenecid on the kinetics of intravitreally injected carbenicillin in rabbits. The half-life of antibiotic in the vitreous of animals with normal eyes was only 5 hr and levels in anterior ocular sites (cornea, aqueous, iris) were low. There findings are consistent with an active retinal transport pump for organic anions. Concomitant intraperitoneal administration of probenecid caused a marked prolongation of the ocular half-life of carbenicillin to 13 hr and augmentation of the levels in anterior sites, as might be anticipated from inhibition of the retinal transport pump. Inflammation produced an intermediate effect that is most readily explained by postulating two opposing consequences: partial inhibition of the transport pump and simultaneous "leakiness" of the normal retinal barrier. There findings suggest that the intraocular kinetics of antibiotics in inflamed eyes are markedly different from those in normal ones and that systemic probenecid significantly augments the intraocular levels of carbenicillin.