Considerable interest has developed in the role of the autonomic nervous system and its effect on blood flow in bone. Theories pertaining to the long-term physiological effect of sympathectomy on blood flow in bone have been controversial. This study employed the radioactive tracer-microsphere technique to investigate the influence of sympathectomy on regional blood flow in bone over time in dogs. Blood flow was measured in fifteen adult dogs, weighing eighteen to twenty kilograms, at one hour, one day, and one, two, three, four, and six weeks after a transperitoneal sympathectomy from the first to the sixth lumbar level. Absolute control flow values were consistent with those that have been reported previously and had lower standard errors. Blood flow in bone in the femoral regions increased by 24.0 to 41.0 per cent. Greater elevations, of 37.0 to 59.0 per cent, were noted in the tibial sections. The largest increases (58.0 to 67.0 per cent) were seen in the metatarsals and proximal phalanges. All elevated values for blood flow returned to control values by six weeks. The data support the hypothesis that surgical sympathectomy exerts a significant yet transient effect on blood flow in bone. Furthermore, the greater increase distally is the first physiological evidence to support previous anatomical work that concluded that the major site of sympathetic tone lies distally in the lower extremity of the dog.