Our laboratory has pioneered the use of bovine serum albumin (BSA) linked to different positions of the ring structure of progesterone to investigate steroid-membrane interactions. The complex can be radioiodinated to demonstrate the existence of specific membrane progesterone binding sites in the rat brain. Not only are these progesterone complexes specific ligands, but they also elicit functional responses in the central nervous system (CNS), particularly in the corpus striatum (CS) where progesterone-BSA conjugates linked at C-3 and C-11 positions (P-3-BSA and P-11-BSA) alter amphetamine-evoked dopamine release. In this communication we will report our current studies that use radioiodinated progesterone-BSA conjugates (P-3-125I-BSA, P-6-125I-BSA, and P-11-125I-BSA) and estradiol-BSA conjugates linked at C-6 position (17 beta-E-6-125I-BSA and 17-E-6-BSA) to demonstrate the existence of specific membrane binding sites for progesterone and estrogen in several regions of the rat brain. In addition, initial studies to isolate and purify these membrane binding sites from digitonin-solubilized P2-membrane fractions by affinity chromatography are reported. The data indicate that these sites are part of a complex membrane receptor for either estrogen or progesterone, the so-called membrane estrogen receptor (mER) and the membrane progesterone receptor (mPR), respectively.