Methodology: The design of this study will be randomized, placebo-controlled, double-blind. Trial duration will be 10 weeks, consisting of a 2-week placebo lead-in period for all subjects, followed by 8 weeks of treatment with either pregnenolone or placebo. The primary cognitive outcome measure will be executive functioning (as assessed by the Tower of London test), and the primary behavioral outcome measure will be PTSD Cluster D symptoms (as assessed by the Clinician-Administered PTSD Scale, CAPS). The investigators will also determine if PREG administration in OEF/OIF Veterans with mild TBI increases downstream ALLO and/or other GABAergic NS levels, and the investigators will identify the specific metabolism profile of PREG following eight weeks of treatment with this neurosteroid. A subset of patients will also receive pre/post neuroimaging (functional magnetic resonance imaging [fMRI], diffusion tensor imaging [DTI], and quantitative susceptibility mapping/susceptibility tensor imaging [QSM/STI]).