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The microvascular effects and hemodynamic events following exposure to normobaric hyperoxia (because of inspiration of 100% O₂) were studied in the awake hamster window chamber model and compared with normoxia. Hyperoxia increased arterial blood Po₂ to 477.9 ± 19.9 from 60.0 ± 1.2 mmHg (P < 0.05). Heart rate and blood pressure were unaltered, whereas cardiac index was reduced from 196 ± 13 to 144 ± 31 ml · min^–1 · kg^–1 (P < 0.05) in hyperoxia. Direct measurements in the microcirculation showed there was arteriolar vasoconstriction, reduction of microvascular flow (83% of control, P < 0.05), and functional capillary density (FCD, 74 ± 16% of control), the latter change being significant (P < 0.05). Calculations of oxygen delivery and oxygen consumption based on the measured changes in microvascular blood flow velocity and diameter and estimates of oxygen saturation corrected for the Bohr …