Agmatine and harmane have been proposed as endogenous ligands of imidazoline receptors. Agmatine has been reported to activate nitric oxide synthetase (NOS) in endothelial cells, so we sought to determine if agmatine or harmane and an analogue of harmane, propyl harmane, produced vasodilatation through an endothelium-dependent mechanism. The experiments were performed in endothelium-denuded and intact rat aortic rings preconstricted with phenylephrine (0.1 microM). Agmatine (0.3-1000 microM), harmane, and propyl harmane (0.3-100 microM) relaxed endothelium-intact rings in a concentration-dependent manner. Removal of endothelium inhibited the relaxant effect of agmatine, harmane, and propyl harmane. The NOS inhibitor L-NIO (100 microM) inhibited the relaxant effect of agmatine and harmane. The I(1)-receptor antagonist AGN (100 microM) partly inhibited the effect of harmane but not that of agmatine. These results suggest that the endogenous imidazoline ligands are capable of stimulating NOS largely by an I(1)-receptor-independent mechanism.