Rabbit blood pressure showed gradual fall by intraventricular guanethidine (5mg). The pressure rise by carotid occlusion was markedly inhibited or abolished by the guanethidine, while it was enhanced by small dose (2mg). The intraventricular guanethidine did not affect norepinephrine pressor effect, but caused marked inhibition or abolishment of depressor action of serotonin.