Substance P (SP) binds to the NK-1 receptor and has been implicated in the transmission of pain as well as in physiological responses such as salivary gland secretion and neurogenic inflammation. Studies in this field have been limited due to the lack of specific antagonists that are not degraded rapidly and are not neurotoxic. However, a recently developed non-peptide SP antagonist, CP-96,345, is specific for the NK-1 receptor. The purpose of this study was to assess the effects of this antagonist on nociception. The tail flick, paw pinch and hot plate tests were used to assess nociception in rats. Following baseline determination of tail skin temperature and analgesiometric tests, the rats received intrathecal injections of various doses of CP-96,345, and pain sensitivity was assessed at several time intervals up to two hours after injection. The ability of CP-96,345 to inhibit SP induced biting and scratching was also assessed. Results from the analgesiometric tests indicated that there were no significant elevations in the latency of tail flick test or the paw pinch thresholds even at 240 micrograms of CP-96,345. The hot plate latency was elevated at the highest dose of antagonist. In addition, there was a significant dose-related elevation in latency on the hot plate test. CP-96,345 also produced a dose-related decrease in tail skin temperature. CP-96,345 did not block SP induced biting and scratching. CP-96,345 and SP were evaluated for their ability to displace 125I-Tyr8-SP from rat spinal cord, brain and submandibular gland membrane fractions. It was found that although the affinity of CP-96,345 was 56 fold lower than that of SP in the brain, the antagonist was nearly as potent as SP in the spinal cord and submandibular gland. The results of this study suggest that, while CP-96,345 binds to the NK-1 receptor in the spinal cord, this receptor is most likely not involved in mediating some types of nociception at the spinal cord level.