Inducible nitric oxide (NO) synthase (iNOS) is believed to contribute to the pathogenesis of endotoxin-induced uveitis (EIU). In the present study, we investigated the inhibitory effects of N G-nitro- l-arginine methyl ester ( l-NAME), a non-selective NOS inhibitor, and S,S′-1,4-phenylene-bis(1,2-ethanediyl)bis-isothiourea (PBITU), a potent and selective iNOS inhibitor, on intraocular NO production in EIU rabbits using an in vivo intraocular microdialysis technique. The flare level in the anterior chamber increased from 1 h after the injection of 100 μg/kg lipopolysaccharide (LPS), and continued to increase for 24 h. Aqueous humor protein concentrations were significantly increased at 24 h after LPS-injection. These changes were significantly reduced by l-NAME (10 mg/kg) and PBITU (1 mg/kg), but not by d-NAME (10 mg/kg). The increase in NO 2 − and NO 3 − levels in the dialysate induced by LPS was significantly inhibited by l-NAME (10 mg/kg) and PBITU (1 mg/kg), but not by d-NAME (10 mg/kg). These results suggest that activation of iNOS may play a key role in the development of EIU, and selective inhibitors of iNOS may have therapeutic applications in the treatment of EIU.