Intravenous norepinephrine (1.0 µg/kg body weight) produced a strikingly uniform pressor effect in rats. The average blood pressure rise was 54 mm Hg, the peak rise occurred in less than 20 seconds, and the blood pressure returned to baseline in about 2 minutes. The aorta was rapidly depleted of potassium (15–18%) by the norepinephrine injection, depletion occurring within 20 seconds (time required for exsanguination) after the peak of the pressor effect and persisting throughout the period of the pressor response. Aorta sodium concentration did not increase concomitantly either in time or in quantity. No other tissue studied (stomach muscle, left ventricle, psoas muscle) showed a parallel alteration in electrolytes. Therefore, injections of norepinephrine appear to selectively affect permeability of vascular smooth muscle to potassium.