Glutamine is a preferred substrate for rapidly proliferating cells. We studied the growth of 31 rat pups from the 6th to the 16th day. The pups were divided into 3 groups. Group 1 was reared with their mother in room air. Groups 2 and 3 were artificially reared on milk substitute via a miniature gastrostomy cannula according to the method previously described (Proc Soc Exp Biol Med 1990, 195:335). They were each further divided into 3 subgroups, a, b and c. Subgroups a were used as controls, while subgroups b and c were supplemented with 1.8 and 9 g/kg/day of glutamine respectively in the milk substitute. Additionally, Group 2 was reared in 80% oxygen and Group 3 was reared in room air. At the end of day 16 the pups were sacrificed. Results: The data were subjected to anova statistical analysis. Overall the body weights of Group 3 pups were greater than Group 1 and group 2 pups. Although the pups supplemented with glutamine (subgroups b and c) tended to have higher body weights than subgroups a the differences were not significant (p>0.05). The weights of the livers, kidneys, and the length of small intestines were greater in Group 2 & 3 than in Group 1. However, the weight of the cortex of the animals in group 2 were smaller than those of groups 1 & 3 (p<0.05). The size of the organs of the glutamine supplemented subgroups were not significantly different from their controls. Conclusions: Pups on an isocaloric diet reared in hyperoxic environment were smaller than those reared in room air. Under the conditions of the experiment and sample size glutamine supplementation did not enhance growth.