Using 3H-labeled derivatives, kinetic parameters of specific binding of progesterone (I) and 16 alpha, 17 alpha-cyclopentanoprogesterone (II) to proteins of the uterus soluble fraction in rats were measured. It was shown that their affinities to proteins are comparable (K 10.5 +/- 2.4 and 6.7 +/- 3.4 nM for (I) and (II), respectively, upon 22 h incubation). The unlabeled compound (II) can displace [3H]progesterone from complexes with the protein with a concentration-independent efficiency corresponding to the ratio of K values for compounds (I) and (II). At the same time, the efficiency of the unlabeled progesterone in the displacement of [3H]compound (II) from protein complexes fell with an increase in the progesterone concentration. The concentration of high-affinity sites of [3H]compound (II) exceeded by 1.5 to 2 times the concentration of sites for [3H]progesterone. Dynamics of dissociation of proteins complexes of [3H]progesterone and 3H]compound (II) had a two-phase character with a decrease in the dissociation rate constants for both phases as the times of exposition of [3H]ligands to proteins grew. The ratio of slow- and fast-dissociating ligand-receptor complexes was thereby unchanged. These data suggest the presence in the rat uterus soluble fraction of two types of proteins differing in the capacity to recognize the additional five-membered ring D' in the steroid molecule.