Specific progesterone binding macromolecules were found in the cytosol of the vagina, the uterine cervix and the mammary glands of castrated, oestradiol primed guinea pigs. The concentration of these receptors was high enough in the vagina and the cervix to allow the study of their characteristics. They were similar to the uterine receptor in their sedimentation coefficient (µ6.5S), their affinity for progesterone (KA µ 5.IO8–1.109M–1), their steroid specificity and their sensitivity to SH-blocking reagents. In not primed with estrogen castrated animals, the vaginal and cervical receptors were present in markedly lower concentrations and their sedimentation coefficient was of about 4.5S. The low concentration of the receptor in the mammary glands in these nonlactating animals did not permit the study of its characteristics. No specific binding was found in the hypothalamus and pituitary cytosol after either in vitro incubation with 3H-progesterone (or 3H-20α-hydroxypregn-4-en-3-one) or in vivo injection of 3H-progesterone. (Endocrinology94: 161, 1974)