The mechanisms of myocardial oxygenation during reactive hyperemia were studied in the beating heart using continuous near infrared (NIR) spectroscopy. In open chest dogs, NIR spectroscopy was used to monitor brief occlusions of the left anterior descending artery. These occlusions produced a precipitous drop in tissue oxygen stores (tHbO2+MbO2), tissue blood volume, and the oxidation level of mitochondrial cytochrome a,a3 . Reperfusion produced a rapid increase in the NIR signals to supranormal levels, followed by gradual return to baseline. When the duration of occlusion was increased from 20 to 120 s, an essentially linear increase was produced in the overshoot areas defined by the NIR signals. Near infrared spectroscopy (NIRS) separated reactive hyperemia into two phases according to the tissue level of deoxyhemoglobin and deoxymyoglobin (tHb+Mb): (1) an early phase during which the tHb+Mb level was supranormal, reflecting enhanced O2 extraction; and (2) a late phase during which the tHb+Mb level was below baseline, reflecting decreased O2 extraction and increased tissue O2 availability. During reactive hyperemia, when O2 availability was maximal by NIR spectroscopy, O2 consumption was elevated but submaximal, indicating that MVO2 was not limited by O2 availability. Cytochrome a,a3 oxidation state also was restored fully. Thus, myocardial oxygenation is highly regulated during reactive hyperemia. Cellular O2 supply and mitochondrial oxidation state are restored early during reactive hyperemia by increased O2 delivery, increases in tissue blood volume and enhanced O2 extraction.