In the estrogen treated guinea pig iv administered progesterone-3H was retained at a seven-fold greater concentration in the uterus than in other tissues. This observation indicated the presence of a progestin concentrating mechanism in uterus and led to the present study. The cytosol prepared from guinea pig uterus 30 min following an iv injection of progesterone-3H contained tritium labeled components which sedimented in sucrose gradients as 7 and 3.5–4s macromolecules. Progesterone-3H also labeled 7 and 3.5–4s binding components when incubated with uterine cytosol in vitro. The protein nature of these binders was suggested by release of progesterone-3H by proteases rather than nucleases. component increased by increasing either the ionic strength or the temperature of the incubation media. The specificity of the cytosol binding proteins for progesterone was demonstrated by competition studies with several steroids. Although cortisol also bound to a 3.5–4s cytosol component, this steroid did not displace progesterone-3H from either of its binding proteins. The progesterone binding activities of guinea pig uterus is stimulated by estrogen treatment. If these components play an essential role in the mechanism of progesterone action, then their stimulation by estrogen would explain the synergistic action of these two steroids on the uterus. (Endocrinology90: 1464, 1972)