Male and female Sprague-Dawley rat siblings (200-350 g) were monitored by phosphorus magnetic resonance spectroscopy for 4 h after moderate (2.8 atmospheres) fluid-percussion-induced traumatic brain injury. Following injury, two of nine male animals died whereas 100% of all female rats (n = 16) died (p < 0.01). Prior to injury, brain free magnesium concentration in males was 0.58 +/- 0.05 mM and in females 0.41 +/- 0.09. After injury, mean brain free magnesium concentration in males declined to 0.32 +/- 0.06 whereas mean brain free magnesium concentration in ventilated females (n = 6) after injury declined to 0.17 +/- 0.03 (p < 0.05). There were no significant differences between groups with respect to any other measured variables. We conclude that female rats are more susceptible to irreversible injury after brain trauma, and that this increased susceptibility to injury may be related to brain free magnesium levels.