The radioactive microsphere technique was used to determine the distribution of cardiac output and regional blood flow in rhesus monkeys before and 10,20,40, and 80 minutes after induction of spinal anesthesia. Five monkeys were studied during low spinal anesthesia (sensory level T10) and five other monkeys were studied during high spinal anesthesia (sensory level T1). Each monkey served as its own control. There was no significant change in regional blood flow during T10 spinal anesthesia. During T1 spinal anesthesia, blood flow (per 100 g of tissue) to kidneys was significantly reduced at 20, 40, and 80 minutes, blood flows to liver and carcass were significantly reduced at 20 and 40 minutes and blood flows to miscellaneous organs (lymph nodes, salivary glands, etc.) were significantly reduced throughout anesthesia. Blood flows to heart, brain, and lower extremity during T1 spinal anesthesia showed only non-significant changes. Vascular resistance in the lower extremity was significantly reduced during both levels of spinal anesthesia, indicating arteriolar dilatation. Also, during both levels of anesthesia, the lungs received an increased proportion of the radioactive microspheres, suggesting increased peripheral arteriovenous shunting of microspheres due to the arteriolar dilatation.