The effects of hormone replacement therapy (HRT) on the central nervous system in postmenopausal women might be mediated by changes in neurosteroid synthesis and/or release. The aim of this study was to evaluate the impact of HRT on the levels of allopregnanolone, a sedative anxiolytic GABA(A) agonist steroid, and dehydroepiandrosterone (DHEA), a GABA(A) antagonist steroid. We evaluated allopregnanolone and DHEA circulating levels after 1, 3, 6, 9 and 12 months of HRT with ten different estrogen or estrogen-progestin molecules, regimens and routes of administration in 186 postmenopausal women. Cortisol, luteinizing hormone, follicle stimulating hormone, estradiol and progesterone levels were also evaluated. Allopregnanolone levels significantly increased during follow-up with all HRT preparations. The addition of progestin molecules (except for 19-nor derivatives) to transdermal estradiol administration alone determined a higher increase in allopregnanolone levels. Transdermal HRT showed a significantly higher percentage change in allopregnanolone levels compared with oral HRT. DHEA levels showed a progressive decline starting from the 3-month follow-up, without significant differences between the transdermal and oral groups, as well as among the ten groups, independently of the presence and type of progestin molecule used. In conclusion, HRT strongly modifies circulating neurosteroid levels in postmenopausal women.