The contributions of gender and gonadal hormones in the cascade of events following brain injury are largely unexplored. We measured cerebral edema following cerebral contusion in rats under three hormonal conditions to address this issue. Normally cycling females exhibited significantly less edema than males, and pseudopregnant females were virtually spared from post-injury edema. Subsequent studies of ovariectomized females, with or without hormone treatment, indicated that the reduction of cerebral edema was associated primarily with the presence of circulating progesterone. We conclude that progesterone has a protective effect on the brain following traumatic injury.