Abstract Nitric oxide (NO) is a highly reactive radical which plays an integral role in physiological and pathophysiological processes. NO is produced endogenously in small amounts by a constitutive NO synthase (cNOS) as a regulator of vascular tone and neurotransmission. NO can also be produced in large amounts by an inducible NOS (iNOS) in response to endotoxin and cytokines, and has been reported to be a mediator of lipopolysaccharide (LPS)-induced uveitis in rats. The purpose of the present study was to investigate the effects of NOS inhibitors with different NOS isoform specificities in the rabbit model of endotoxin-induced ocular inflammation. LPS and /or inhibitors of NOS, N G-nitro- l-arginine methyl ester (L-NAME) and aminoguanidine (AG), were injected intravitreally and the eyes observed by slit lamp for 24hr. Coinjection of LPS with L-NAME inhibited anterior inflammation in rabbits. Iridal hyperemia (IH) and aqueous flare (AF) were completely abolished in eight out of nine rabbits in a dose-dependent manner. In addition, total cell counts were significantly suppressed (7393 ±697 vs. 325 ±188, P<0.05) and aqueous protein levels were reduced to near control levels (25 ±0.75 vs. 1.72 ±0.36, P<0.05). Similar suppression was seen with AG (cell counts=351 ±246 and proteins=3.1 ±1.2). Administration of L-NAME 0.5hr after LPS injection suppressed inflammation to a lesser extent than coinjection. In contrast, administration of L-NAME 6hr after LPS injection was not inhibitory, and in fact significantly increased cellular infiltration. However, AG given 6hr after LPS had a remarkably different effect, since it significantly decreased both protein extravasation and cellular infiltration into the aqueous humor. In fact, our results suggest that cNOS may play a greater role in the earlier stages of this developing inflammatory response. These results extend others ' observations that NO is a key mediator in uveitis, that induction of iNOS plays a critical role in experimental uveitis, and suggest that NO has a complex role in the ocular inflammatory process. Inhibitors of NOS can abort the LPS-induced inflammatory response if administered early enough, but could potentially exacerbate an established inflammatory episode.