Immune responses are suppressed in males, but not in proestrous females, after trauma-hemorrhage. Testosterone and 17β-estradiol appear to be responsible for divergent immune effects. There is considerable evidence to suggest sex steroid hormone involvement in immune functions. As formation of active steroid depends on the activity of androgen- and estrogen-synthesizing enzymes, expression and activity of 5α-reductase, aromatase, and 3β- and 17β- hydroxysteroid dehydrogenases were determined in spleen and T lymphocytes of male and proestrous female mice after trauma-hemorrhage. All of the enzymes were present in spleen, specifically in T lymphocytes. 5α-Reductase expression and activity increased in male T lymphocytes, whereas aromatase activity, but not expression, increased in female T lymphocytes. Increased 5α-reductase activity in male T lymphocytes is immunosuppressive because of increased 5α-dihydrotestosterone synthesis, whereas in females increased aromatase activity triggering 17β-estradiol synthesis is immunoprotective. This study also demonstrates the importance of 17β-hydroxysteroid dehydrogenase oxidative and reductive functions. The immunoprotection of proestrous females is associated with enhanced reductase function of the enzyme. In males, decreased expression of oxidative isomer type IV, which impairs catabolism of 5α-dihydrotestosterone, probably augments immunosuppression. This study provides evidence for the involvement of intracrine sex steroid synthesis in gender dimorphic immune responses after trauma-hemorrhage.