Blood flow was measured in mature and immature dogs by means of tracer microspheres. Microspheres of 15 micron were found to be the most suitable size in the dog. Total body nonentrapment in the awake, standing dog is likely to be less than 10%. Cortical bone flood flow, devoid of periosteum and marrow, is 2.5 ml/100 g/min in mature dogs and 7.0 ml/100 g/min in immature dogs, P < 0.005. Blood flow in cancellous bone is greater than that in cortical bone in mature (P < 0.001) and immature (P < 0.02) dogs. Flow is different in different regions of a long bone because of different proportions of cortical and cancellous bone, probably because of interrelationships of function (surfaces undergoing remodeling) and, therefore, of energy metabolism and blood flow.