To test for the presence of CRH receptors, we prepared myometrial membranes and performed binding studies using [I-125]tyr-o-CRH as a ligand. The binding was found to be pH, time, temperature, and divalent cation concentration dependent and was fully reversible on addition of 1 muM unlabeled ovine CRH. In both tissues, there was a single, specific, homogenous, high affinity population of CRH receptors. Scatchard analysis of the specific binding sites revealed dissociation constants of 250-300 and 30-60 pM for the nonpregnant and pregnant myometrium, respectively. This compares with dissociation constants of 130 pM (rat anterior pituitary receptor) and 100 pM (human CRH-binding protein). This would mean that in the nonpregnant state, the equilibrium for binding is in favor of the binding protein, but during the later stages of pregnancy, the change in affinity of the receptor alters the binding in favor of the myometrial receptor. 741, 1993)