SR 140333 (1-[2-[3-(3,4-dichlorophenyl)-1-(3-isopropoxyphenylacetyl) piperidin-3-yl]ethyl]-4-phenyl-1-azonia-bicyclo[2.2.2]octane , chloride), a potent non peptide ligand of the substance P (SP) NK1 receptor subtype with high affinity for NK1 receptors from both rat cortical membranes and human IM9 cells (Ki = 0.02 nM and 0.01 nM, respectively) was studied in vivo on various effects induced by NK1 agonists in rats and mice. SR 140333 given intraperitoneally (i.p.) in mice antagonized dose-dependently and in a stereoselective manner the scratching responses induced by intracerebroventricular SP and septide (ID50 = 0.73 and 0.08 mg/kg, respectively) and the turning behavior elicited by intrastriatal SP and septide (ID50 = 0.07 and 0.06 mg/kg, respectively). This compound had little effect on the scratching responses and the turning behavior elicited by [Sar9, Met(O2)11]-SP. When SR 140333 was coadministered with the peptide agonist, the compound reduced the scratching responses elicited by SP, [Sar9, Met(O2)11]-SP and septide injected intrathecally (i.t.) in mice (ID50 = 72.0, 64.3 and 52.5 ng i.t., respectively). SR 140333 antagonized the salivation induced by SP, [Sar9, Met(O2)11]-SP and septide in rats (ID50 = 0.13, 0.18 and 0.09 mg/kg i.p., respectively). SR 140333 abolished the facilitation of the tail-flick reflex induced by noxious heat in rats (total reversal at 0.06 mg/kg, i.p.). This compound was also found to inhibit the turning behavior induced by intrastriatal apomorphine in mice (ID50 = 0.1 mg/kg, i.p.). In conclusion, these results indicate that SR 140333 behaves as a potent, selective and centrally active NK1 receptor antagonist.