The perfusion of the heart, kidneys, adrenals, stomach, intestines, brain and musculature under normobaric hyperoxia was studied in anaesthetized mini pigs using radioactively labelled microspheres. After a 2-hour control period using room air, the influence of normobaric hyperoxia on the perfusion of the organs was measured at 2-hour intervals. A significant increase in the perfusion of tissue was found in the kidneys, adrenals, gastro-intestinal tract and brain after 4-6 hours of exposure. No change in perfusion could be demonstrated for the myocardium. Values for the skeletal musculature (diaphragm and musculus psoas) were widely scattered, but an increase could be suspected. The increase in tissue perfusion at constant blood pressure and pulse rate must be interpreted as an expression of vasodilation. These results are contrary to the majority of statements in the literature, which mainly report vasoconstriction under normobaric hyperoxia.