Blood flow during "resting conditions" and during noradrenaline infusion were studied by the labeled microsphere technique in dimethylbenz(a)anthracene-induced mammary tumors, skin, muscle, and lung in the rat. Intratumoral distribution of flow was studied by autoradiography of spheres trapped in the vascular beds of the tumors. Histological examination was performed and correlated to the blood flow data. Mean blood flow to the tumors during "resting conditions" was relatively high (49 ml/min/100 g tissue) but was substantially decreased (5 ml/min/100 g tissue) during noradrenaline infusion which produced a pressure elevation of 35 mm Hg. Thus, vascular resistance of the tumor tissue increased dramatically. Cardiac output increased, but total systemic resistance was unchanged. Vascular resistance in muscle was unchanged in contrast to an increase seen in skin. Shunted systemic blood flow to the lungs and bronchial arterial flow decreased indicating reactivity of abnormally large arteriovenous passages in the tumors. Poorly differentiated tumors had a higher vascular resistance than did well-differentiated tumors. Autoradiography revealed a nodular flow distribution with a slight tendency of higher perfusion in the periphery of these tumors.