Effects of intraventricular norepinephrine (NE) on rabbit blood pressure and heart rate were investigated. 1) Blood pressure was little affected by small doses of NE (below 500μg) but showed marked rise by 1 mg. 2) Heart rate was decreased by intraventriccular NE (200~500μg). One mg of NE caused less pronounced bradycardia than with smaller doses. The bradycardia could not be observed in vagotomized or atropinized animals. 3) Intraventricular NE potentiated reflexive bradycardia produced by 5-hydroxytryptamine. 4) Cord-sectioned rabbit showed different responses; the smaller doses (100~200μg) produced transitory bradycardia and depression of blood pressure, which followed by tachycardia and pressure rise. The transitory bradycardia and depressor effects were not observed in cord-sectioned and vagotomized rabbit. 5) Treatment of animals with reserpine, guanethidine and hexamethonium changed the effects of intraventricular NE on blood pressure, i.e., in these cases the smaller doses of NE caused maked elevation of blood pressure. 6) From these observations it was inferred that central NE caused stimulation of cardioinhibitory and vasomotor center. The former seemed to be more sensitive to NE than the latter. Susceptibility of the vasomotor center to NE seemed to be influenced by peripheral sympathetic tone.