 Valproic acid has been found to be an antagonist of the androgen and progesterone receptors, and hence as a nonsteroidal antiandrogen and antiprogestogen, at concentrations much lower than therapeutic serum levels.In addition, the drug has been identified as a potent aromatase inhibitor, and suppresses estrogen concentrations.These actions are likely to be involved in the reproductive endocrine disturbances seen with valproic acid treatment.