The whole-cell voltage clamp technique was used in order to study the effects of Angiotensin II (Ang II) on the slow inward current and the K+ outward current in single aortic cells of the rabbit. Angiotensin II (10(-8) M) increased the slow inward Ba ++ current, and the addition of an antagonist of Ang II, [( Leu8] Ang II, 10(-8)M) rapidly reversed the effect of Ang II on IBa. Angiotensin II (5 x 10(-8)M) greatly decreased K+ current and the Ang II antagonist reversed this effect. Thus, it is quite possible that the decrease of IK and the increase of Isi in aortic single cells by Ang II may explain a part of the vasoconstrictor effect of this hormone in vascular smooth muscle.