BACKGROUND: Although evidence of systemic vasoconstriction has been reported both in animal models and in humans, the regional hemodynamic effects of hyperbaric hyperoxia have not been well characterized. METHODS: In the present study, we report the effects of hyperoxia (normobaric and hyperbaric) on simultaneous measurements of cardiac and regional hemodynamics in the chronically instrumented conscious dog. RESULTS: Hyperbaric hyperoxia (202 kPa) produced significant decreases in heart rate (12%) and cardiac output (20%) and a significant increase in systemic vascular resistance (30%). Carotid artery blood flow decreased significantly (18%) whereas coronary, hepatic, renal and mesenteric flows remained unchanged. CONCLUSIONS: Our data show that the hyperoxic vasoconstriction is limited to the cerebral and peripheral vascular beds. Additionally, blood flow to major organs is well preserved in the face of hyperoxia-induced decreases in cardiac output. Consequently, we postulate that a redistribution of blood flow from peripheral vascular beds (e.g., skin, muscle, bone) to major organs occurs during hyperbaric hyperoxia.