Binding sites for IGF-I, insulin, and GH were localized by in situ binding of (125)I-labelled hormones to the different compartments of the sow ovary. Binding sites for IGF-I were detected in oocytes, granulosa and thecal cells of healthy and atretic follicles as well as in the antrum and the stroma. Competition of (125)I-labelled IGF-I with IGF-I, insulin and an analogue of IGF-I (Long R(3)IGF-I), which allowed discrimination between binding to binding proteins from binding to type-I receptors, suggested that type-I receptors were present in granulosa cells of healthy follicles, whilst binding in other compartments was mainly due to binding proteins. Binding of insulin was revealed in oocytes, granulosa and theca interna cells of healthy preantral and antral follicles, and, to a lesser extent, in theca externa and stromal cells, and was still observed in granulosa cells of atretic follicles. Labelling with (125)I-labelled bovine GH was demonstrated in oocytes, granulosa cells, theca interna cells, and, although less intense, in theca externa and stromal cells. It disappeared in granulosa cells during atresia. Binding sites for GH were detected at all follicular stages, from preantral to preovulatory stages, but the intensity of labelling in granulosa cells was more intense in preantral than in large follicles. These data support the participation of insulin, GH and IGF-I in oocyte maturation, follicular growth and stromal cell function in swine.