Marked plasticity in GABAA receptor signalling occurs in adult oxytocin neurons of the supraoptic nucleus (SON) through the modulation of GABAA receptor alpha subunits during pregnancy. The present studies were undertaken to examine the potential mechanisms underlying this plasticity. In vivo microdialysis experiments in conscious rats revealed that no significant changes in extracellular GABA concentrations occurred within the SON over the last two days of pregnancy and the time of parturition itself. In situ hybridization studies examined the effects of gonadal steroid manipulation upon the GABAA receptor subunits expressed by SON neurons (alpha1, alpha2, beta2 and gamma2 subunits) and demonstrated that cellular levels of the alpha1 subunit were increased following 8 days oestrogen and progesterone treatment. Estrogen alone or allopregnanolone, the progesterone derivative, had no effect on alpha1 subunit mRNA expression in the SON. Immunocytochemical experiments demonstrated progesterone receptors in many neural populations but not within the SON of late pregnant rats. These studies indicate that alterations in endogenous GABA release within the SON are unlikely to be responsible for the GABAA receptor plasticity exhibited by oxytocin neurons in late pregnancy. Rather, data demonstrate that the fluctuating concentrations of progesterone during pregnancy act indirectly on SON neurons to modulate alpha1 subunit mRNA expression. Together, these experiments provide evidence for the ligand-independent induction of GABAA receptor plasticity in the adult brain by progesterone.