Although immune functions are markedly depressed in males and not in proestrous females following traumahemorrhage (T-H), the mechanisms responsible for the divergent responses remain unknown. Because sex steroids modulate the activation of p38, our aim was to determine whether differences in the activation of p38 by phosphorylation (p38-P) might contribute to the sex-dimorphic immune response following T-H. The effects of testosterone and estradiol on the activation of p38 were also examined. Intact male mice (C3H/HeN), castrated males treated with vehicle, 5α-dihydrotestosterone (DHT), or 17β-estradiol, and proestrous females were subjected to trauma (i.e., midline laparotomy) and hemorrhagic shock (35 ± 5 mmHg for 90 min and resuscitation) or sham operation. At 2 h thereafter, splenic (SMφ) and peritoneal macrophages (PMφ) were harvested and cultured (with 10 μg/ml LPS), and Western blot analysis was carried out for quantification of p38 and p38-P. Sex, testosterone and estradiol plasma levels, and T-H did not alter the constitutive expression of p38 in SMφ and PMφ. In contrast, the activated form of p38 (p38-P) was markedly increased in SMφ and PMφ from female shams compared with male shams. Moreover, the phosphorylation of p38-P increased in males after T-H, whereas it decreased in females under those conditions. Castration before T-H prevented the increase in p38-P in males. Castrated animals treated with DHT displayed increased p38-P following T-H, whereas 17β-estradiol had no effect on p38-P in castrated mice. Thus 1) sex influences the activation of p38 MAP kinase, 2) DHT is responsible for the increased activation of p38 in male mice, and 3) this sex-specific activation of p38 might be responsible for the sexually dimorphic immune response following T-H.