PIP: A specific progesterone receptor of myometrial cytosol from the rhesus monky is described. Characterization of the receptor by sucrose density gradient centrifugation revealed 2 peaks at 4s and 7.5s. The 4s peak seen in all groups (castrate; castrate plus estrogen treated; castrate plus estrogen and progesterone treated) contained little specific progesterone binding but the 7.5s peak, seen only in the estrogen-treated animal, was specific for progesterone. Competition studies revealed the reeceptor affinities to be: progesterone 100, 5alpha dehydroprogesterone 81.9 melengestrol acetate 72.5, norgestrel 53, desoxycorticosterone 25.9, 5beta-dihydroprogesterone 1.2, and 17 hydroxyprogesterone less than 1. Receptor levels measured from Scatchard plot analysis of of equilibrium data were 7 fM/mg cytosol protein (castrate), 45.2 fM/mg (p less than .01, estrogen treated), and 10.5 (estrogen plus progesterone treated). The association constant (approximately 5 x 10(-9)M) was similar in all 3 groups.