Abstract: The effects of Citrus aurantium L(Ⅰ) and one of its active ingredients N-methyl-tyramine (Ⅱ) on vascular resistance were studied with roller pump in anethetized dogs. Their actions on receptor were analysed by antagonists. Intra-arterial and intra-venous injection of Ⅰ and Ⅱ increased the renal and cerebral vascular resistance signi-ficantly, which could be antagonized by phentolamine. This suggests that α-receptor sti-mulation is involved. Ⅱ exerted a biphasic effect on femoral artery resistance. This is explained on the differences of distribution in a and β-adrenoceptors.Under constant renal infusion, the transient diuretic action observed after injection of Ⅰ or Ⅱ is independent of the change of renal blood flow.Receptor action of the positive inotropic effect of Ⅱ in guinea pig's auricle strip in vitro, and rabbit heart in situ was attributed mainly to the stimulation of β-adren-ergic receptor.In reserpinized rats and dogs the hypertensive effects of Ⅱ were significantly dee-reased. Pressor tachyphylaxis of Ⅱ similar to tyramine was observed in rats. Assay of adrenergic mediator content in rat myocardium showed significant decrease in nor-epinephrine content after repeated injections of Ⅱ and tyrarnine, while dopamine remained unchanged. It was concluded that the cardiovascular effects of Ⅱ similar to tyramine are resulted from liberation of intrinsic adrenergic mediator, while Ⅰ contains synephrine as well, which shares a direct agonist action on α-receptors.