A cytoplasmic receptor is necessary for target tissues to respond to steroid hormones. Since the hypothalamic-pituitary unit is responsive to progestins, it might be expected to have a progesterone receptor. The clinical evaluation of receptors located in the central nervous system is not possible, so the clinical investigator must extrapolate from accessible tissues. To determine whether such extrapolation is valid, binding affinities and specificity of progesterone receptor sites in the cytoplasm of bovine uterus, ovary, hypothalamus, and pituitary were measured. In each particular tissue the receptors showed similar binding characteristics with progesterone and R5020. It is concluded that in a given species the receptor for progesterone is similar in all target tissues and probably results from the expression of the same gene.