The following experiments were designed to investigate the role of estrogen in central autonomic nuclei on autonomic tone and reflex control of heart rate. Male Sprague-Dawley rats were anesthetized with sodium thiobutabarbital (100 mg/kg) and instrumented to record blood pressure and heart rate. Efferent vagal and renal nerve activities were recorded and used to assess changes in parasympathetic and sympathetic tone, respectively. The cardiac baroreflex was evoked using a single bolus injection of phenylephrine (0.1 mg/kg) both before and following either intrathecal injection of estrogen (0.5 microM; 1 microl) to influence sympathetic preganglionic neurons of the intermediolateral cell column or bilateral injection of estrogen (0.5 microM; 100 nl/side) into the nucleus tractus solitarius, rostral ventrolateral medulla or nucleus ambiguus. The cardiac baroreflex was significantly enhanced following both intrathecal and medullary injections of estrogen. Efferent vagal nerve activity was significantly increased following injection of estrogen into the nucleus tractus solitarius, nucleus ambiguus and the intrathecal space. Renal sympathetic nerve activity was significantly depressed following injection of estrogen into the nucleus tractus solitarius, rostral ventrolateral medulla and the intrathecal space. In all cases, simultaneous injection of estrogen with the selective estrogen receptor antagonist, ICI 182,780 (1 pM) blocked all previously observed changes in baroreflex function and autonomic tone. These results demonstrate a role for estrogen in the reflex control of heart rate and as a central modulator of autonomic tone in male rats.