We have investigated the source of central adenosine important in modulating the cardiovascular response to hypothalamic defense area (HDA) stimulation in alpha-chloralose-anesthetized rats. Microinjections of an ecto-5'-nucleotidase inhibitor, alpha,beta-methylene ADP (alpha,beta-meADP), were made into caudal nucleus of the solitary tract (cNTS) and rostral ventrolateral medulla (RVLM), and its effects on HDA-evoked responses were observed. Stimulation of HDA evoked an increase in arterial pressure and a secondary rise in arterial pressure after stimulation ceased. There was also an increase in heart rate and hindlimb blood flow. alpha,beta-meADP had no effect on resting levels of arterial pressure, heart rate, and hindlimb blood flow when injected into the cNTS or RVLM. alpha,beta-meADP also had no effect on the HDA-evoked tachycardia and increase in muscle blood flow. However, alpha,beta-meADP reduced the primary increase in arterial pressure evoked by HDA stimulation when microinjected into the cNTS. In contrast, alpha,beta-meADP reduced the secondary increase in arterial pressure when microinjected into the RVLM. From these results, we suggest that at least part of the adenosine released centrally during HDA stimulation is derived extracellularly from ATP metabolism.