Three psychological active principles from the seeds of Peganum harmala L., harmine, harmaline and harmalol, showed vasorelaxant activities in isolated rat thoracic aorta preparations precontracted by phenylephrine or KCl with rank order of relaxation potency of harmine > harmaline > harmalol. The vasorelaxant effects of harmine and harmaline (but not harmalol) were attenuated by endothelium removal or pretreatment with a nitric oxide (NO) synthase N^ω-nitro-L-arginine methyl ester. In cultured rat aortic endothelial cells, harmine and harmaline (but not harmalol) increased NO release, which was dependent on the presence of external Ca²⁺. In endothelium-denuded preparations, pretreatment of harmine, harmaline or harmalol (3 - 30 μM) inhibited phenylephrine-induced contractions in a non-competitive manner. Receptor binding assays indicated that all 3 compounds interacted with cardiac α₁-adrenoceptors with comparable affinities (K_i value around 31 - 36 μM), but only harmine weakly interacted with the cardiac 1,4-dihydropyridine binding site of L-type Ca²⁺ channels (K_i value of 408 μM). Therefore, the present results suggested that the vasorelaxant effects of harmine and harmaline are attributed to their actions on the endothelial cells to release NO and on the vascular smooth muscles to inhibit the contractions induced by the activation of receptor-linked and voltage-dependent Ca²⁺ channels. The vasorelaxant effect of harmalol was not endothelium-dependent.