Blood flow to mammary tumor MT-W9B in minimally disturbed, unanesthetized inbred WF rats was measured with radioactively labeled 25-mu (diameter) microspheres. The distribution of the blood flow estimates was not described by a normal probability density function but could be normalized with a natural log transformation of the data. The relative blood flow to the tumor tissue was always greater than that to the surrounding normal tissue and was dependent on the tumor size, transplantation site (axillary vs. inguinal), and physiologic state of the animal. The growth of this tumor did not alter the blood flow to the adjacent normal tissues. However, the cardiac output and coronary blood flow increased with increasing tumor size. Two blood flow estimates could be performed on the same animal within a time-span of 15 minutes.