Androgens have been implicated as the causative factor for the post-injury immune dysfunction in males; however, it remains unknown whether androgens directly affect macrophages. To study this, male mice were sham-operated or subjected to trauma (i.e., midline laparotomy) and hemorrhagic shock (MAP 30 ± 5 mmHg for 90 min and then resuscitated). The mice received the 5-reductase inhibitor 4-hydroxyandrostenedione (4-OHA) prior to resuscitation. Plasma TNF-, IL-6 and IL-10 levels were elevated following trauma-hemorrhage and normalized by 4-OHA. TNF- and IL-6 production by splenic macrophages (sM) was decreased after injury, whereas Kupffer cell (KC) production of these mediators was enhanced. 4-OHA normalized cytokine production. Androgens suppressed cytokine production by sM from hemorrhaged mice, whereas it enhanced TNF- and IL-6 production by KC. Addition of 4-OHA, in vitro, normalized cytokine production by cells treated with testosterone, but had no effect on dihydrotestosterone (DHT)-treated cells. These results indicate that androgens directly affect macrophage function in males following trauma and hemorrhagic shock and that the intracellular conversion of testosterone to DHT is of particular importance in mediating the androgen-induced effects.