[Show abstract][Hide abstract] ABSTRACT: The present study evaluated the effects of histamine 10(-2) M on longitudinal preparations of rat portal vein. It was observed that histamine 10(-2) M induced relaxation of rat portal vein preparations pre-contracted with phenylephrine 10(-4) M. On the other hand, no pharmacological effects were observed in preparations not pre-contracted. The observed histamine-induced relaxing effect was absent in preparations pre-contracted with KCl (120 mM) or in the presence of depolarizing nutritive solution. However, the histamine-induced relaxation was still present in the endothelium-removed preparations. The histamine-induced relaxation also was not prevented by astemizole (10(-6) M, 10(-5) M and 10(-4) M), cimetidine (10(-5) M, 10(-4) M and 10(-3) M) or thioperamide (10(-6) M, 10(-5) M and 10(-4) M), selective antagonists H(1), H(2) and H(3), respectively. The presence of L-NAME 10(-4) M or L-NAME 10(-4) M plus indomethacin 10(-5) M also did not prevent the histamine-induced relaxation observed in rat portal vein. Thus, the histamine-induced relaxation observed in rat portal vein appears to involve a non-endothelial hyperpolarizing mechanism independent of H(1), H(2) and H(3) receptors.