In small animals such as the rabbit the ratio of eye size to body size is much larger than it is in humans. Accordingly, periocular injection of antibodies in this animal model results in significantly higher serum concentrations than does a comparable does in a human. To assess the effect of the systemic drug component on ocular penetration, we compared the levels of gentamicin in ocular tissues and fluids of the rabbit following injection of 20 mg by subconjunctival or intramuscular routes. Blood levels of gentamicin were similar with the two routes of administration. In normal rabbit eyes, no antibiotic was detectable in the vitreous humor for the first 3 hr after either subconjunctival or intramuscular injection. By 6 hr, low vitreous levels of drug were detectable after subconjunctival, but not after intramuscular, injection. Because these concentrations were so close to the threshold of sensitivity of the assay, it was not clear that the difference between the routes was significant in normal eyes. In infected eyes, the concentrations of gentamicin in the vitreous were similar after subconjunctival and intramuscular injection. These data indicate that the penetration of gentamicin in to the infected vitreous humor of rabbits after subconjunctival injection could be attributed as well to hematogenous carriage as to direct penetration. The problem in distinguishing systemic from direct transport with periocular injections may be inherent in any small animal model in which the ratio of eye size to body size is high.