Angiotensin II (ANG II) is a potent vasoconstrictor of splanchnic and renal resistance vessels. Because ANG II increases during exercise and blood flow in the splanchnic and renal circulations decreases, we tested the hypothesis that ANG II plays a role in arterial blood pressure and regional blood flow responses to treadmill running in the miniswine. Consequently, 11 pigs were instrumented with epicardial electrocardiogram leads and left atrial and aortic catheters to assess mean arterial blood pressure (MAP), heart rate (HR), myocardial contractility, cardiac output, and regional blood flow during treadmill running. Each animal exercised for 20 min at 80% of its maximal HR reserve. Exercise was performed in the absence and presence of the ANG II AT1 receptor antagonist losartan (15-20 mg/kg). ANG II AT1 receptor blockade attenuated the MAP and systemic vascular resistance responses to dynamic exercise but had no effect on cardiac output, HR, or myocardial contractility. In addition, blood flow increased and/or regional vascular resistance decreased in the heart, kidneys, stomach, small intestine, and colon, whereas the reverse occurred in the skin and spleen. These data suggest that ANG II contributes to the increase in MAP and redistribution of cardiac output associated with dynamic exercise.