Tetrahydroisoquinoline (THI) alkaloids can be considered as cyclized derivatives of simple phenylethylamines. Many of them, especially with 6,7-disubstitution, demonstrate a relatively high affinity for catecholamines. Present study examines the pharmacological action of limited series of THI, using rats' isolated atria and aorta. In addition, a [3H] prazosin displacement binding study with THI compounds was performed, using rat brain homogenates to investigate whether these probes have ㄁-adrenoceptor affinity. We also compared the vascular relaxation potency of these probes with dobutamine. YS 49, YS 51, higenamine and dobutamine, concentration-dependently, relaxed endothelium-denuded rat thoracic aorta precontracted with phenylephrine (PE, 0.1 ㄍM) in which pEC50 were 5.56⁑0.32 and 5.55⁑0.21, 5.99⁑1.16 and 5.57⁑0.34, respectively. These probes except higenamine also relaxed KCl (65.4 mM)-contracted aorta. In isolated rat atria, all THIs and dobutamine increased heart rate and contractile force. In the presence of propranolol, the concentration response curves of YS 49 and YS 51 shifted to the right and resulted in pA2 values of 8.07⁑0.84 and 7.93⁑0.11, respectively. The slope of each compound was not deviated from unity, indicating that these chemicals are highly competitive at the cardiac ㄂-adrenoceptors. YS 49, YS 51, and higenamine showed ㄁-adrenoceptor affinity in rat brain, in which the dissociation constant (Ki) was 2.75, 2.81, and 1.02 ㄍM, respectively. It is concluded, therefore, that THI alkaloids have weak affinity to ㄁1-adrenoceptors in rat aorta and brain, respectively, while these probes show relatively high affinity for cardiac ㄂-adrenoceptors. Thus, these chemicals may be useful in the treatment of congestive heart failure.