We investigated whether changes in nitric oxide (NO) release might be responsible for the modulation by glucocorticoids of the pressor response to i.p. injection of interleukin-1beta (IL-1beta) in freely moving rats. In such rats, IL-1beta (10 microgram/kg) induced a biphasic pressor response, with a rise in the plasma concentration of NOx (NO2(-) and NO3(-): metabolites of NO) during the second phase. Systemic pretreatment with an exogenous glucocorticoid, dexamethasone (0.5 mg/kg), enhanced the second phase of the pressor response and completely suppressed the increase in plasma NOx. Treatment with Nomega-nitro-L-arginine methyl ester (L-NAME, a nonspecific NO synthase inhibitor), enhanced the pressor response while attenuating the increase in plasma NOx. After bilateral adrenalectomy, IL-1beta induced a smaller pressor response, but a larger increase in plasma NOx; dexamethasone reversed these changes. Our results suggest that endogenous NO moderates the pressor response to IL-1beta in freely moving rats, and that glucocorticoids enhance the IL-1beta-induced pressor response at least in part by reducing endogenous NO release.