IGF-I has been implicated as a local and/or systemic regulator of ovarian function by acting alone or as an amplifier of the actions of gonadotrophins, thereby influencing follicular growth and ovulation. In the sheep it is uncertain as to whether IGF-I can fulfil an autocrine or paracrine role, since mRNA expression and peptide synthesis have not previously been demonstrated. Using in situ hybridization with ovine and human riboprobes, IGF-I mRNA was found in all major steroidogenic cell types of the sheep ovary, namely the granulosa, theca and luteal cells and, to a lesser extent, the stroma. IGF-I mRNA was found to be expressed in the granulosa and thecal cells of early antral follicles and thereafter in these cell types as the follicles increased in diameter. Evidence for IGF-I mRNA expression in preantral follicles could not be demonstrated. No obvious differences in the levels of expression were observed in ovaries recovered during the luteal phase, follicular phase, anoestrus or after exogenous FSH treatment. IGF-I expression was confirmed by Northern blotting and PCR. These findings are consistent with the notion that IGF-I may have an autocrine or paracrine role in enhancing ovarian function in the sheep.