Pregnenolone is the major steroid precursor in humans. It is also a "neurosteroid" and possesses intrinsic behavioral and brain effects in animals, affecting the GABA(A) and other receptors. In two preliminary studies, we sought to characterize its tolerability and psychotropic effects in humans. In Study 1, 17 normal volunteers received pregnenolone and placebo for 4 weeks each (15 mg PO per day x2 weeks followed by 30 mg PO per day x2 weeks, vs. placebo x4 weeks) in a within-subject, double-blind, cross-over design, with a 4 week drug-free washout period separating the two arms. Subjects' behavioral responses were assessed at the beginning and end of the 4-week pregnenolone arm and the 4-week placebo arm. Pregnenolone was generally well-tolerated but, by itself, had no significant effects on mood, memory, self-rated sleep quality or subjective well-being. In Study 2, 11 subjects from Study 1 received a single dose of diazepam (0.2 mg/kg PO) immediately following completion of Study 1 in order to assess, in a between groups design, the impact of 4-weeks' pre-treatment with pregnenolone (N=5) vs. placebo (N=6) on the acute sedative, amnestic and anxiolytic effects of this benzodiazepine. Pregnenolone-pretreated subjects showed significantly less sedation following diazepam (p<0.03); this effect was clinically apparent. Diazepam's amnestic effects were non-significantly attenuated, and ratings of anxiety were unaffected. These pilot data, based on small samples, raise the possibility that chronically administered pregnenolone antagonizes certain acute effects of benzodiazepines and may enhance arousal via antagonist or inverse agonist actions at the benzodiazepine/GABA(A) receptor complex. Further larger-scale studies, utilizing a broader range of doses and experimental conditions, are warranted.