This study evaluated the role of nitric oxide (NO) in spinal cord nociceptive transmission during peripheral inflammation evoked by formalin injection into the rat paw, using N omega-nitro-L-arginine (N-Arg), an NO synthase inhibitor. Male rats were prepared with intrathecal (IT) catheters. To quantify the formalin response, the instances of "flinching behavior" were counted at 5-min intervals for 60 min. IT N-Arg depressed the flinching behavior in a dose-dependent manner when N-Arg was administered 10 min before the formalin injection. This N-Arg effect was reversed with L-arginine but not D-arginine. We conclude that NO plays an important role in nociceptive transmission in the spinal cord during the formalin test.