IGF-I receptors have been identified and characterized in a variety of tissues. In this study receptors for IGF-I in the rat uterine tissue were identified and characterized. We have demonstrated IGF-I receptors in crude uterine membranes by binding and cross-linking experiments. IGF-I binding to the rat uterine membranes displayed time, temperature and pH dependance, and optimal binding conditions were achieved by 20 h of incubation at 4°C, at a pH of 7.8. Uterine IGF-I binding sites were specific for binding IGF-I peptide and demonstrated less than 100 × lower affinity for insulin. The binding was reversible and Scatchard analysis indicated presence of a single class of binding sites with an apparent dissociation constant of 1.68 ± 0.24 nmol/l and Bmax of 0.82 ± 0.1 pmol/mg protein. During estrogen treatment, sensitization and decidualization there was an overall increase of membrane proteins in the uterus and a relative decrease of IGF-I receptors per unit of protein. When expressed on a per uterus basis, there was a progressive increment in total IGF-I binding in estradiol-treated, sensitized, and decidualized uterus compared with controls. These data indicate a possible role for IGF-I in uterine cell multiplication and further differentiation to decidual cells in response to deciduogenic stimuli.