It was recently shown that L-glutamine inhibits vascular nitric oxide (NO) production in vitro. The present study investigated the effect of glutamine enriched enteral diets on in vivo NO production in the rat. Nitrate, the stable end-product of NO production, was measured in plasma and 24 h urine collections in glutamine supplemented rats (6.25%, 12.5% and 25% w/w) and compared to the effect of isocaloric, nitrogenous control diets. Glutamine supplementation increased plasma levels of glutamine (up to 91%), arginine (up to 17%) and citrulline (up to 54%). After 1 week of glutamine supplementation plasma nitrate levels were significantly reduced by 50% compared to control (P < 0. 0001); irrespective of the amount of supplementation. No further decrease was observed after 2 weeks of feeding. No differences in daily urinary losses were found between the groups. These results point to an in vivo inhibitory effect of glutamine supplemented enteral feeding on NO production.