Background: Previous experiments have shown that warming hypoxic infants reduces total peripheral vascular resistance. This suggests that the usual vasoconstriction of less essential vascular beds during hypoxia may be reduced and that the normal redistribution of blood flow to more vital organs may be compromised. Objective:Evaluate the effect of body warming during hypoxia on the distribution of blood flow. Methods: The fluorescent microsphere technique was used to compare regional blood flow in 1-month-old rabbits during systemic hypoxia (10% inspired O2) with (n = 9) and without (n = 10) body warming. Blood flow was measured in brain, stomach, small intestine, hindlimb muscle, skin, and kidneys. Arterial blood pressure, whole-body O2 consumption, arterial blood O2 saturation and blood gases were also measured. Measurements and Main Results: In hypoxia all animals decreased body temperature (–2°C). With hypoxia blood flow increased to brain and hindlimb muscle; decreased to stomach, small intestine, and kidneys, and was unchanged in skin. The increase in brain-blood flow maintained O2 delivery at normoxic levels. Rewarming to the normoxic body temperature significantly changed blood flow in hypoxia. Brain blood flow increased by 102 ± 30% (mean ± SEM) thereby increasing O2 delivery by 50 ± 23% above normoxic values. Blood flow also increased to skin, stomach, and small intestine. However, O2 delivery to these tissues remained below normoxic levels. Conclusions: Warming during hypoxia may impose an additional cardiovascular demand. The changes in the pattern of blood flow distribution with mild warming during hypoxia support the hypothesis that warming represents a significant heat stress.