Reserpine, serotonin, norepinephrine, 5-hydroxytryptophan and 3,4-dihydroxyphenylalanine had qualitatively the same cardiovascular effects when they were injected into a lateral ventricle or into the cisterna magna. All lowered arterial pressure, usually caused bradycardia despite prior section of the vagus nerves, and caused marked inhibition of the pressor response to occlusion of the common carotid arteries. Essentially the same results were obtained in unanesthetized as in anesthetized dogs. On a weight basis, norepinephrine was the more active amine tested and 5-hydroxytryptophan was more active than serotonin and 3,4 dihydroxyphenylalanine. Reserpine injected centrally had an effect equivalent to that of 20 or more times larger dosage given intravenously. All drugs were more active against the response to carotid occlusion than against the reflex pressor response to stimulation of a cut central end of a sciatic or vagus nerve. Decreased afferent electric activity of the carotid sinus nerve accompanied lowering of arterial pressure following central injection of small dosage, or intravenous injection of large dosage, of 5-hydroxytryptophan, indicating that the cardiovascular effects are not due to a direct action of 5-hydroxytryptophan on carotid sinus baroceptors.Central injection of an amine oxidase inhibitor, beta-phenylisopropylhydrazine, markedly augmented and prolonged the cardiovascular effects of the amines and of reserpine as well.These results are all consistent with, though they do not validate, the premise that the acute cardiovascular effects of reserpine are mediated centrally by serotonin and/or norepinephrine, either released from a bound and inactive to a free and active form, or formed by decarboxylation of their respective amino acids.