This study attempted to determine the correlation between oxygen consumption (VO2), pulmonary-capillary blood flow (QEPC), and oxygen delivery (DO2) by rebreathing and invasive techniques obtained over a range of hemoglobin concentration and cardiac output.Twenty mongrel dogs were instrumented with central arterial and venous catheters to determine DO2by thermodilution cardiac output and standard formulas. The animals were administered isoproterenol in doses that increased DO2and subsequently were serially phlebotomized by 30%, 40%, and 50% to decrease DO2. All animals were studied using a rebreathing technique to determine noninvasively VO2, QEPCand DO2.Sixteen dogs completed the experimental protocol. A correlation analysis was carried out for VO2, QEPC, and DO2obtained by the rebreathing and invasive methods. Thermodilution cardiac output increased from 3.91 ± 1.77 L/min at baseline to 8.19 ± 2.50 L/min during isoproterenol infusion. Hemoglobin varied from 12.21 ± 1.26 gm% at baseline to 5.21 ± 1.36 g% at 50% phlebotomy. Over this range of conditions, significant correlations were obtained between rebreathing VO2and invasive VO2(r = 0.80, p < 0.0001), rebreathing QEPCand invasive QEPC(r = 0.79, p < 0.0001), and rebreathing DO2and invasive DO2(r = 0.82 p < 0.0001).These data demonstrate that the rebreathing technique can be used to monitor oxygen metabolism over wide ranges of DO2.