The presence of mu-opioid receptors and endomorphins has been demonstrated in the general area encompassing the rostral ventrolateral medullary pressor area (RVLM). This investigation was carried out to test the hypothesis that endomorphins in the RVLM may have a modulatory role in regulating cardiovascular function. Blood pressure (BP) and heart rate (HR) were recorded in urethane-anesthetized male Wistar rats. Unilateral microinjections of endomorphin-2 (0.0125-0.5 mmol/L) into the RVLM elicited decreases in mean arterial pressure (MAP; 16-30 mmHg) and HR (12-36 bpm) which lasted for 2-4 min. Bradycardia was not vagally mediated. The effects of endomorphin-2 were mediated via mu-opioid receptors because prior microinjections of naloxonazine (1 mmol/L) abolished these responses; the blocking effect of naloxonazine lasted for 15-20 min. Unilateral stimulations of aortic nerve for 30 sec (at the frequencies of 5, 10 and 25 pulses/sec; each pulse 0.5 volts and 1 msec duration) elicited depressor and bradycardic responses. These responses were significantly attenuated by microinjections of endomorphin-2 (0.2 and 0.4 mmol/L). The inhibitory effect of endomorphin-2 on baroreflex responses was prevented by prior microinjections of naloxonazine. Microinjections of naloxonazine alone did not affect either baseline BP and HR or baroreflex responses. These results indicate that endomorphin-2 elicits depressor and bradycardic responses and inhibits baroreflex function when injected into the RVLM. These effects are consistent with known hyperpolarizing effect of opioid peptides on RVLM neurons.