Abstract Groups of mice were injected intraperitoneally with L-tryptophan, 1 mg/gm body weight, and maintained at room temperature (22° C) or exposed to acute heat stress (36° C). In the heat-stressed animals, plasma tryptophan levels were lower at each time interval studied when compared with room temperature controls; however, both hepatic tryptophan and tryptophan oxygenase activity levels were not significantly different. In mice which had been preexposed to heat for one hour prior to tryptophan administration, plasma levels were decreased further, yet hepatic substrate concentration and enzyme activity were unaffected. Hence, acute exposure to environmental heat reduces vascular uptake of the amino acid, but as a result of rapid enzyme induction, hepatic tryptophan and tryptophan oxygenase activity are unaffected.