The relationship between blood flow and oxygen transport was studied in five calves with chronic nonpulsatile biventricular bypass. Seven days was allowed for recovery from the effects of anesthesia and operation; the natural heart was then fibrillated. Pump flows were maintained at nominal rates of 90, 100, or 120 ml.kg-1.min for 1 week each, with the sequence varied from experiment to experiment. Venous and arterial blood samples were taken at rest for blood gas analysis. Serum lactate analysis was done twice a week, on the third and seventh days after each pump flow change. Serum catecholamine levels were assayed on the seventh day of each flow rate. Progressive exercise tests were also conducted during each test segment. Basal oxygen consumption of a 4-month-old calf was 6.3 +/- 0.3 ml.kg-1.min-1. The mixed venous oxygen tension decreased when pump flow rate was reduced (29.6 +/- 1.0, 28.3 +/- 1.2, and 23.8 +/- 0.9 mm Hg at 120, 100, and 90 ml.kg-1.min-1 of pump flow, respectively), and oxygen extraction increased linearly when pump flow rate was reduced. Hemoglobin concentration significantly affected oxygen extraction rate. Serum lactate concentration increased significantly at a 90 ml.kg-1.min-1 perfusion compared with concentrations at other pump flow rates (7.81 +/- 2.42 mEq/L at 90 ml.kg-1.min-1 vs 0.71 +/- 0.19 and 0.73 +/- 0.81 mEq/L at 100 and 120 ml.kg-1.min-1, respectively; p < 0.01, analysis of variance, Scheffe F test). Maximum oxygen extraction during exercise was 78%. These results suggest that a critical flow level between 90 and 100 ml.kg-1.min-1 maintains oxidative metabolism in the calf with chronic nonpulsatile flow. The resulting oxygen delivery was slightly higher than that indicated in the literature. Maximal oxygen extraction was normal.