Exogenous glucocorticoids markedly increase the in vivo uptake and utilization of glutamine by the intestine. Since glutamine is the major oxidative fuel for the small intestine, we investigated whether glucocorticoids induce changes in the specific activity of the enzymes that mediate glutamine degradation (glutaminase) and synthesis (glutamine synthetase). Male Sprague-Dawley rats received a 7-day elemental diet. On Day 5, animals were randomized to one of four groups and received either saline (Control) or one of three doses of dexamethasone im: 0.1 mg/kg (Lodex); 0.3 mg/kg (Middex); or 0.6 mg/kg (Hidex). Forty-eight hours later jejunal and colonic segments were assayed for protein, glutaminase, and glutamine synthetase activity. A stress dose of dexamethasone (Hidex) produced a significant increase in both jejunal and colonic glutaminase specific activity (P less than 0.02 vs Control and P less than 0.05 vs Control, respectively). These data suggest a mechanism whereby glucocorticoids increase the intestinal utilization of glutamine by increasing the specific activity of intestinal glutaminase.