Effects of peroxynitrite (ONOO-), its stable product 3-nitrotyrosine (NT) and sodium nitroprusside (SNP) on isolated rat anococcygeus muscle were investigated. Administration of 0.1-1.0 mM ONOO- or 0.01-100.0 muM SNP produced concentration-dependent relaxations on the phenylephrine (2 muM)-precontracted muscle. Time courses of these relaxations to ONOO- and SNP were not similar and decomposed ONOO- caused a biphasic response composed of an initial contraction followed by a relaxation, NT (0.01-0.1 mM) either incubated for 20 min prior to precontraction or given during pre contraction plateau did not attenuate precontraction or ONOO--induced relaxations. Results of the present study demonstrate that ONOO- relaxes rat anococcygeus muscle specifically while its stable metabolite NT has no effect. (C) 2000 Elsevier Science Inc. All rights reserved.