Pregnancy increases affinity of [3H]muscimol binding to GABAA receptors in the rat forebrain. Post-partum, the receptor affinity is further increased concomitantly with a reduction of the receptor density. These changes may result from an action of endogenous placental and adrenal steroids, tetrahydroprogesterone and tetrahydrodeoxycorticosterone, which in vitro behave as allosteric agonists of GABAA receptors. The alterations may contribute to the psychological phenomena associated with pregnancy and the puerperium.