The aim of this study was to evaluate the effects of centrally administered neuropeptide Y (NPY) on muscular rigidity induced by fentanyl in Sprague-Dawley rats. They were anesthetized with ketamine (120 mg/kg, i.p.) and their lungs were mechanically ventilated. Intravenous administration of fentanyl (100 microg/kg) consistently evoked a significant increase in the electromyographic activity (EMG) recorded from the sacrococcygeus dorsalis lateralis muscle. This implied muscular rigidity was appreciably attenuated by intracerebroventricular administration of NPY (4 nmol/2.5 microl). Microinjection of NPY (40 or 160 pmol/50 nl) into the bilateral locus coeruleus (LC) elicited an inhibition of the EMG activation induced by fentanyl in a dose-dependent manner. Microinjection of 160 pmol NPY plus antiserum against NPY (NPY(Ab), 1:20) into the LC failed to suppress fentanyl-induced muscular rigidity. However, this implied muscular rigidity was not affected by NPY plus normal rabbit serum (NRS, 1:20). In addition, NPY(Ab) or NRS per se had no substantial effect on the rigidity. Microinjection of NPY (160 pmol) into the areas adjacent to the LC did not attenuate the rigidity. Our results suggest that the centrally administered NPY attenuated fentanyl-induced muscular rigidity by acting at the LC, but endogenous NPY may not be involved in this process.