To test the role of peripheral vasoconstrictor tone in the efficient use of a limited O2 supply, three groups of anesthetized dogs were ventilated with 9.1% O2 until circulation failed. Two groups were alpha-blocked with phenoxybenzamine and one of those was volume expanded with dextran to restore blood pressure. After O2 utake was lowered in hypoxia, O2 uptake was linearly related to O2 delivery (cardiac output X arterial O2 content) with r = 0.94. The slope of that line was mathematically identical to the extraction ratio and it increased from 0.54 at 10 min to 0.87 at the end of hypoxia (r = 0.99). In both alpha-block groups O2 extraction remained constant with time, and O2 extraction in each alpha-block group was significantly less (P less than 0.01) than in the unblocked group. As further evidence of better O2 extraction, mixed venous partial O2 pressure was significantly less in the unblocked group, 6.2 +/- 3.4 Torr vs. 10.6 +/- 3.2 and 9.9 +/- 2.4 Torr with alpha block (P less than 0.01). Results after alpha-block indicated that a vigorous vasoconstrictor tone during hypoxia conserved O2 by promoting greater extraction by the tissues.