Blood-flow distribution was determined in pentobarbital-anesthetized male B6C3F1 mice, using intracardially administered 141Ce-radiolabelled microspheres (10 micro Ci). The proportion of cardiac output and blood flow per g tissue was determined in 18 organs and tissues, including the nasal cavity, lobes of the liver, individual kidneys and sections of the intestinal tract. To provide comparative data, blood-flow distribution was also determined in pentobarbital-anaesthetized male Fischer 344 rats. In general, there was close agreement between the distribution of blood flow between these two rodent species. The bladder and large intestine of mice appear to receive more, while the testes receive less, of the cardiac output than in rats. Rat liver and kidney, however, appear to receive approximately twice the amount of blood on a weight basis as in the mouse.