Vascular perfusion characteristics have been studied in dimethylbenz(a)anthracene-induced rat mammary neoplasia and compared with those of skin, skeletal muscle, salivary gland, kidney, spleen, uterus, and brain by means of an artificial perfusion technique. Perfusion of tissues and organs was measured by the microsphere tracer technique. This procedure makes possible a detailed hemodynamic analysis of several tissues under controlled conditions, in this study maximum vascular relaxation, without confounding endogenous vasoregulation. The maximal perfusion capacity, i.e., during smooth muscle relaxation, of tumors and various tissues was related to perfusion pressure at three levels by means of three differently labeled microspheres. Tumors, especially large ones, have a low maximum perfusion capacity, i.e., high vascular resistance, compared to most other tissues. For the tumors, a relatively high perfusion pressure is required to open up the otherwise collapsed vascular network.