Anesthesia, surgical trauma, heparinization, priming volume composition, and temperature control of the heart-lung machine individually affect carbohydrate, protein, or lipid metabolism during cardiac operations. The impact of some of these factors on glucose and insulin regulation was assessed before, during, and after normothermic cardiopulmonary bypass in nondiabetic patients with use of a servo-controlled insulin delivery system. With a glucose-free prime, cardiopulmonary bypass induced a slight hyperglycemia but no endogenous insulin response, suggesting a partial inhibition of insulin secretion. Nonetheless, insulin release could be stimulated by exogenous glucagon. A glucose load in the priming fluid led to marked and persistent hyperglycemia without commensurate insulin release. Elevated stress hormone levels, a concomitant reduction of insulin release and insulin action, and a depression of peripheral glucose utilization, as demonstrated by glucose clamp experiments, contributed to these perturbations of glucose and insulin metabolism. Although the metabolic alterations observed are not critical in routine cardiac operations, they may become clinically significant in postoperative states with unusual persistence of stress conditions.