The authors had previously observed a deleterious cerebrovascular effect of prolonged hypothermia in primates and cats. In this study they examined the systemic as well as cerebral hemodynamic and metabolic effects of 24 hours of hypothermia in the dog. With decreases in temperature to 29 C, cardiac output (Q) and whole-body oxygen consumption (VO2) initially decreased 52 and 42 per cent, respectively. Thereafter, despite a stable temperature, both Q and VO2 continued to decrease, and at 24 hours values were 7 and 28 per cent of control, respectively, Cerebral blood flow (CBF) and cerebral oxygen consumption responded similarly. At 24 hours inhomogeneous perfusion of both brain and skeletal muscle was observed. With rewarming, cardiovascular collapse with severe tissue hypoxia and acidosis developed; CBF became grossly inadequate, resulting in depletion of brain energy stores.