In contrast to L-glutamine, lipid emulsions are routinely administered to patients receiving nutritional support. The provision of fat during intravenous feeding is essential, but the potentially toxic byproducts of fatty acid oxidation may have adverse metabolic consequences. In the present study, we have examined the effect of L-glutamine, an inhibitor of fatty acid oxidation, on the development of defective blood glucose regulation caused by a 48-hour infusion of 10% intralipid in rats. Male Sprague-Dawley rats (200-290 g) were anesthetized with sodium pentobarbital, the right femoral vein cannulated, and baseline blood samples were taken. Each rat was placed in a metabolic cage with access to water, in the presence or absence of rodent chow. Two hours after waking, the rats were infused with 10% intralipid with either saline (control), 2% L-glutamine, or 2% L-alanine. After 48 hours, all animals were sacrificed and blood samples were again obtained. The mean +/- SEM plasma glucose levels before and after lipid infusion at the rate of 1 mL/hr in control rats fed ad libitum, were 125 +/- 13 and 170 +/- 5 mg/dL (p < 0.01, n = 7). Similarly, plasma free fatty acids (FFA) in these animals rose from 0.74 +/- 0.11 to 1.34 +/- 0.32 mmol/L (p < 0.05). Plasma insulin levels also increased from 337 +/- 44 to 1278 +/- 88 pg/mL (p < 0.01). Reduction of intralipid dose infusion did not prevent insulin resistance characterized by hyperglycemia and hyperinsulinemia. However, addition of L-glutamine to the high-dose lipid infusion with chow feeding prevented changes in plasma glucose, insulin levels, and FFA but not triglyceride levels. Also, glutamine but not alanine supplementation in intralipid infused rats without chow feeding prevented changes in plasma glucose, insulin, and malondialdehyde levels. In conclusion, these data show that glutamine supplementation during intravenous lipid administration in rats prevents the development of impaired glucose regulation associated with hyperlipidemia.