Progesterone is readily reduced in humans to its A-ring metabolites, allopregnanolone (3α-hydroxy-5α-pregnan-20-one) and pregnanolone (3α-hydroxy-5β-pregnan-20-one). The latter have been reported to have anxiolytic, hypnotic and anesthetic actions when administered to laboratory animals and (or) humans. Consequently, we measured allopregnanolone and pregnanolone in 18 healthy females, ages 18-25, at the time of peak plasma progesterone following an oral dose of micronized progesterone (1,200 mg) in a double-blind, placebo-controlled study. The plasma levels of the parent steroid and metabolites were compared with changes in mood, cognition, and motor performance following progesterone administration. We observed good correlations between plasma progesterone and plasma allopregnanolone (r = 0.85), plasma pregnanolone (r = 0.81) and the combined metabolites (r = 0.92). Plasma allopregnanolone was significantly correlated with measures of fatigue, confusion and immediate recall, and these correlation coefficients were somewhat greater than those for plasma progesterone and these same behavioral measures. Significant changes in fatigue, delayed verbal recall and symbol copying were experienced by subjects who achieved high levels (≥ 95.55 nmol/l) of these anxiolytic metabolites, while those with lower metabolite levels reported no negative effects. These data suggest that allopregnanolone and pregnanolone may contribute to or mediate the observed behavioral effects of progesterone.