The purpose of this study was to determine the retention and ocular distribution of subconjunctivally administered nanoparticles and microparticles. Fluorescent polystyrene particles (carboxylate modified, negatively charged) of various sizes (20 nm, 200 nm and 2 microm; Fluospheres, dose 400 microg) were administered to male Sprague-Dawley rats by subconjunctival injection under anaesthesia. The disposition of the particles in the periocular and ocular tissues was studied for up to 60 days by quantifying the particle amounts using liquid extraction followed by spectrofluorimetric analysis. The effect of dose on the particle disposition was investigated with a 40-microg dose of the particles. The effect of an increase in surface hydrophobicity was evaluated for the 20 and 200 nm particles at 1 day post administration. Following periocular administration, penetration into the ocular tissues was negligible for the carboxylate-modified microparticles as well as nanoparticles. Almost the entire dose of the 200 nm and 2 microm particles was retained in the periocular tissue at 60 days post-administration. The 20 nm particles disappeared rapidly from the periocular tissue with 15 and 8% of administered dose remaining after 1 and 7 days, respectively. The 20 nm particles could not be detected in the periocular tissue at 60-days post-administration. An increase in the surface hydrophobicity did not affect the periocular retention of 200 nm particles but elevated that of the 20 nm particles, at the end of day 1. It was concluded that subconjunctivally administered 200 nm and larger particles can be almost completely retained at the site of administration for at least two months. Periocular administration of particulate systems of this size would likely be useful as sustained drug delivery systems.