Cerebral edema is a serious side effect of traumatic brain injury. We have previously established that progesterone injections, initiated within 1 h after cortical contusion injury, reduced edema when assessed 3 days later. To determine how rapidly progesterone can reduce edema, male and female rats were given the hormone 1 h after damage to the medial frontal cortex, and edema levels were assessed between 2 h and 7 days postinjury. Progesterone decreased edema with 6 h of the injury and continued to be effective for the duration of treatment. In addition, we assessed whether progesterone injections are effective when delays are imposed between injury and initiation of treatment. Male and female rats received progesterone after postinjury delays 6, 24, or 48 h. Progesterone was effective in reducing edema when treatment was delayed until 24 h after injury.