We have recently shown that progesterone promotes myelin formation in peripheral nerves of rodents. In this study, we demonstrate the presence of progesterone receptors (PR) in primary cultures of rat Schwann cells, the glial cells of the PNS, prepared from sciatic nerves of 4-5 days old rats. After 3 weeks of culture, the presence of PR was measured by whole cell assay after incubating living cells for 1 h at 37 degrees C with [3H]-Organon 2058 as a ligand, and about 5000 specific binding sites per cell were found. In contrast to the PR of rat glial cells from the central nervous system (CNS), which is induced by estrogens, treatment of Schwann cells with estradiol did not increase the PR-binding, even after exposure of cells to high doses of estrogen under various culture conditions. Progesterone receptors were also visualized in Schwann cells by indirect immunofluorescence staining with a monoclonal anti-PR antibody. Again, treatment of the cells with estradiol did not increase the immunofluorescence staining of the PR. Specific PR binding was also measured in sciatic nerves of adult female rats. Cytosol was prepared and labeled with [3H]-Organon 2058 for 15 h at 2 degrees C. After treatment with dextran-coated charcoal, specific ligand binding was about 30 fmol/mg cytosolic protein. When castrated adult female rats were treated with estradiol (20 micrograms EB/day for 3 days), no PR-induction was observed in the cytosol of sciatic nerves. In contrast, PR-binding sites in cytosols prepared from pituitary gland and uteri of the same animals were significantly increased by the estrogen.