1. The proportion of the cardiac output delivered to the skeleton and individual bones has been determined in rats of differing ages and weights with radioactive microspheres.
2. There was a decrease in the blood flow to the tibia—fibula, jaw, femur, pelvis, humerus, radius and ulna, and scapula with increase in age and weight of rats, but no change was seen in blood flow to the ribs, spine, skull or whole skeleton.
3. The proportion of the cardiac output delivered to the whole skeleton was approximately 3%.
4. The derived values for skeletal blood flow agree well with estimates by other authors using the technique of bone uptake and blood clearance of 45Ca.