An assessment was made of the role of insulin-like growth factor-I (IGF-I) in the growth of granulosa and thecal cells of the pre-ovulatory follicles in the ovary of the domestic hen (Gallus domesticus). The presence of IGF-I in granulosa and thecal tissues was demonstrated by radioimmunoassay (RIA) of tissue extracts and by immunocytochemical analysis of cultured cells. Both studies showed that immunoreactive IGF-I was present in granulosa and thecal tissues. Reverse-transcription, polymerase chain reaction and Southern blotting analysis showed that IGF-I mRNA was present in total RNA extracted from granulosa and thecal tissue of the four largest follicles in the follicular hierarchy. Cell culture systems for chicken granulosa and thecal cells were established and used to determine the effects of IGN-I alone, and with gonadotrophins, on DNA synthesis using the incorporation of [3-H] thymidine into the cell as an index. The results of these studies showed that IGF-I stimulated DNA synthesis in both cell types in a dose-dependent manner. FSH and LH were shown to stimulate steroidogenesis, as measured by RIA, in thecal and granulosa cells respectively, but neither gonadotrophin stimulated DNA synthesis in thecal cells. LH but not FSH was shown to have a dose-dependent stimulatory effect on DNA synthesis in granulosa cells; when IGF-I and LH treatments were combined their stimulatory effects were synergistic. Serum factors were also shown to act synergistically with IGF-I in this respect. The effects of IGF-I were found to depend on follicle size, as did the effects of LH on granulosa cells. The synergistic actions of IGN-I and LH with respect to DNA synthesis in granulosa cells were independent of follicular size. The production of IGF binding proteins (IGFBP) by granulosa and thecal cell cultures was demonstrated using Western ligand blotting. A range of IGFBPs were shown to be produced by both cell types; when the patterns were compared, both were found to produce four proteins of the same size, but granulosa cells produced an additional protein not detected in thecal cell conditioned medium. The stimulation of cultures with IGF-I resulted in increased production of IGFBPS by granulosa but not by thecal cells; stimulation with LH had no effect on IGFBPs by granulosa cells.