The hypothesis that GH may affect gonadal function by increasing local levels of the GH-dependent somatomedin C/insulin-like growth factor I (IGF I) was tested. Ovine GH (200 micrograms) was injected into immature, hypophysectomized, estrogen-treated female rats; animals were sacrificed 8 or 12 h later. Renal and ovarian homogenates were acid extracted and chromatographed over Sephadex G-50. Eight h after GH injection, 3.6 to 6.4-fold increases in immunoreactive IGF I (IR-IGF I) levels were observed in either ovarian or renal extracts subjected to acid chromatography. Twelve h after GH treatment, IR-IGF I levels remained elevated, but were lower than after 8 h. In neither case could IR-IGF I levels be accounted for by serum contamination. IR-IGF I eluted with an apparent mol wt near that of synthetic human IGF I in both kidney and ovary. Thus, GH can directly increase ovarian and renal tissue IR-IGF I levels in vivo. Taken with previous observations showing a direct gonadotropin-enhancing effect of IGF I on rat granulosa cells in vitro, our results support the hypothesis that GH may affect ovarian differentiation by inducing the local production or accumulation of IGF I, providing evidence for a novel intraovarian paracrine control mechanism.