Cardiac output, regional tissue and arterial tumor blood flow were studied in rats, with 99Tcm labelled microspheres using a reference sample simultaneously drawn as the spheres were injected. Both cardiac output and regional blood flow can be estimated in absolute values (ml × min–1 × g–1). Cardiac output and normal tissue blood flow were not affected by tumor growth in the liver or subcutaneously. In small tumors total blood flow was inversely proportional to tumor size, a finding similar in all 3 tumors studied. A relation found in liver as well as in subcutaneously implanted tumors. Blood flow in subcutaneous tumors was greater than muscle or skin blood flow. BP sarcoma had higher blood flow than the hepatoma and the adenocarcinoma. In big tumors the peripheral blood flow was larger than that in the centre of the tumor.