The binding of tritiated proesterone to the cytoplasmic progestogen receptors from oestrogen-primed sheep endometrium and myometrium has been investigated. The binding characteristics of the progesterone receptors from both sources were determined in the supernatant fractions obtained after high speed centrifugation of the myometrial and endometrial homogenates. High affinity binding proteins with identical association constants for progesterone (1 times 10-9 M-1) were detected in both endometrium and myometrium. The concentration of binding sites was also of the same order of magnitude in both tissues. After centrifugation on surcrose gradients, these binding proteins were shown to have similar sedimentation constants, 7S in a gradient containing no added KC1 and 4S in a gradient containing 0.4 M KC1. The binding peaks from both sources could be abolished by heating the cytosol at 60 degrees C for half an hour. The proteinaceous nature of the binding materials was demonstrated by incubation of the endometrial and myometrial cytosols with pronase, DNase and RNase: the binding was totally eliminated by the proteolytic action of pronase whereas DNase and RNase had no effect. The ligand specificity of the two progesterone binding proteins was studied using a competitive protein-binding technique. Both the endometrial and the myometrial receptor proteins were shown to bind only steroids which are known potent progestogens. In addition, the relative affinities of the binding proteins for 37 steroidal compounds closely resembled each other. Thus, the physiochemical and binding data obtained show that a very close similarity exists between the progesterone-binding proteins in the endometrium and myometrium of ovariectomized sheep after oestradiol treatment.