The possible functional relation between changes in brain and plasma concentrations of neurosteroids and the plasticity of gamma-aminobutyric acid type A (GABA(A)) receptors in the brain during pregnancy and after delivery was investigated in rats. The concentrations in the cerebral cortex and plasma of pregnenolone as well as of progesterone and its neuroactive derivatives allopregnanolone (3alpha-hydroxy-5alpha-pregnan-20-one) and allotetrahydrodeoxycorticosterone (5alpha-hydroxy-3alpha,21-diol-20-one) increased during pregnancy, peaking around day 19, before returning to control (estrus) values immediately before delivery (day 21). In the postpartum period, steroid concentrations in plasma and brain did not differ from control values. The densities of [3H]GABA, [3H]flunitrazepam, and t-[35S]butylbicyclophosphorotionate (TBPS) binding sites in the cerebral cortex also increased during pregnancy, again peaking on day 19 and returning to control values on day 21; receptor density was decreased further 2 days after delivery and again returned to control values within 7 days. These changes were accompanied by a decrease in the apparent affinity of the binding sites for the corresponding ligand on day 19 of pregnancy. The amount of the gamma2L subunit mRNA decreased progressively during pregnancy, in the cerebral cortex and hippocampus, returned to control value around the time of delivery and did not change in the postpartum period. On the contrary, the amount of alpha4 subunit mRNA was not modified during pregnancy both in the cerebral cortex and hippocampus whereas significantly increased 7 days after delivery only in the hippocampus. No significant changes were apparent for alpha1, alpha2, alpha3, beta1, beta2, beta3 and gamma2S subunit mRNAs. Administration of finasteride, a specific 5alpha-reductase inhibitor, to pregnant rats from days 12 to 18 markedly reduced the increases in the plasma and brain concentrations of allopregnanolone and allotetrahydrodeoxycorticosterone as well as prevented both the increase in the densities of [3H]flunitrazepam and [35S]TBPS binding sites and the decrease of gamma2L mRNA normally observed during pregnancy. The results demonstrate that the changes in the plasticity of GABA(A) receptors that occur in rat brain during pregnancy and after delivery are related to the physiological changes in plasma and brain concentrations of neurosteroids.