Phosphorus magnetic resonance spectroscopy was used in conjunction with neurologic motor function tests to assess the effects of estrogen on biochemical and neurologic outcome following traumatic brain injury in male and female rats. Male (n = 18) and female (n = 18) rats were randomly assigned into three groups, and 4 h prior to injury received either 17 beta-estradiol (144 micrograms/kg intraperitoneally), equal volume vehicle (30% ethanol in saline), or no treatment. Traumatic brain injury was induced at 2.8 atm using a fluid percussion injury device, and animals monitored for 4 h using phosphorus magnetic resonance spectroscopy to determine brain intracellular pH, free magnesium concentration and cytosolic phosphorylation potential. Males treated with estrogen demonstrated a significant improvement in free magnesium concentration, and slightly improved values of cytosolic phosphorylation potential after trauma when compared to controls. There was also a significant improvement in post-traumatic motor function at 1 week after trauma. In contrast, estrogen treatment in females lowered cytosolic phosphorylation potential after trauma, but did not affect free magnesium concentration after trauma. Mortality in all female groups was significantly worse than in males. We conclude that estrogen is protective in males, but exacerbates brain injury in females through effects mediated by estrogen receptor binding.