The inhibitory effect of female sex steroid hormones on the binding of [35S]t-butylbicyclophosphorothionate [35S]TBPS to the chloride ion channel receptor in different forebrain areas of the female rat proved to be of a differential nature. The in vivo administration of estradiol and estradiol+progesterone were responsible for substantially lower chloride ion channel receptor levels in brain areas that contain elevated steroid receptors, such as the medial preoptic area, the cortico-medial amygdala nucleus, the vertical limb diagonal band-medial septal nucleus and the cortex lamina V. The administration of progesterone alone reduced receptor levels in the oriens-pyramidalis CA1 layer of the hippocampus, caudate putamen, cortex lamina VI (brain areas that contain little if any steroid receptors) and in the lateral and basolateral amygdala nucleus (brain sites that contain noninducible progesterone receptors). On the basis of the progesterone-inhibitory activity on the chloride ion channel receptors, it was important to investigate whether progesterone per se or whether the potent progesterone metabolites 3 alpha-hydroxy-5 alpha-dihydroprogesterone (3 alpha, 5 alpha-THP) and 3 beta-hydroxy-5 beta-dihydroprogesterone (3 beta,5 beta-THP) were involved in the binding level changes, and to establish the specific brain sites where these steroid effects occur. In fact, in vitro addition of the 5 alpha-reduced progesterone metabolite produced even greater depressive effects on [35S]TBPS binding not only in the same brain areas as the in vivo progesterone replacement therapy but also in some sites that provided significant receptor level changes following the sequential administration of estradiol+progesterone. However, when the 5 beta-reduced metabolite was tested on the binding of [35S]TBPS to the chloride ion channel receptor, only the basolateral amygdala nucleus, the cortex lamina VI and the dorsolateral septal nucleus exhibited changes. Because the steroid-mediated chloride ion flux is regulated in a GABA-dependent manner, we next checked for the type of GABA effects on the chloride ion channel receptor levels and found that GABA not only intensified the 3 alpha,5 alpha-THP inhibitory effects but, together with this progesterone metabolite, was also involved in binding changes in the vertical limb diagonal band-medial septal nucleus. It is interesting to note that the GABA effects on 5 beta-metabolite-induced receptor changes were not of the enhancing type, but tended, rather, to be inhibitory.(ABSTRACT TRUNCATED AT 400 WORDS)