BACKGROUND Glutamine is a conditionally essential amino acid that is critical for many basic cellular processes. Its supplementation has been found to be beneficial during several critical illnesses. This study examines the effects of increased glutamine availability on insulin-mediated glucose homeostasis in vivo in multicatheterized conscious canines (n = 5). METHODS Two weeks before the study, catheters were placed in the femoral artery and the portal, hepatic, femoral, and renal veins for blood sampling and in the splenic vein for intraportal infusion of insulin and glucagon. Doppler probes were placed to measure blood flow. The metabolic study consisted of equilibration, basal, and experimental periods during which [3-3H]glucose was infused to measure glucose kinetics. During the 5-hour experimental period, a hyperinsulinemic-euglycemic clamp was performed by infusing somatostatin, basal glucagon, fivefold basal insulin, and glucose to maintain euglycemia. The experimental period was divided evenly into two subperiods performed in random order: (1) i.v. glutamine infusion (0.72 mmol kg(-1) h(-1)) and (2) i.v. saline infusion. RESULTS With glutamine, the glucose required to maintain euglycemia was increased 46% over saline (6.8 +/- 1.0 to 9.9 +/- 1.7 mg kg(-1) min(-1). In addition, whole-body glucose production and utilization were increased by 1.4 and 4.6 mg kg(-1) min(-1), respectively. Finally, the increase in whole-body glucose utilization was manifested by increased hepatic and hindlimb glucose utilization. CONCLUSIONS Increased glutamine availability blunted insulin's action on glucose production and enhanced insulin-mediated glucose utilization with the changes in utilization being threefold greater than the changes in production. Thus parenteral glutamine has potential benefit as a nutrient adjuvant during clinical situations associated with insulin resistance.