Studies were designed to determine the effect of naloxone on sympathetic nerve activity and blood pressure and, also to investigate the central effect of naloxone in relation to the baroreflex system in alpha-chloralose-anesthetized cats. Following intravenous injection of naloxone (2 mg/kg), preganglionic splanchnic nerve (PSN) activity significantly increased in parallel to the increase in blood pressure. Set gain (mmHg-1) of carotid sinus baroreflex together with the operational range (mmHg) was increased after naloxone. Intraventricular (4th) injection of naloxone (0.2 mg/kg) produced identical responses in blood pressure and PSN activity as well as an altered set gain and range of the baroreflex. Additionally, the pressor response to carotid hypotension and medullary pressor area stimulation were augmented during naloxone activation but not the pressor response to posterior hypothalamus stimulation. The depressor responses to stimulation of both carotid sinus nerve and medullary depressor region were facilitated by naloxone. These data suggest that naloxone has an effect on the central autonomic blood pressure regulatory circuits which participate in the carotid sinus baroreflex system.