It has recently been shown that androgens increase the growth of immature follicles in the primate ovary. In the present study the effect of androgens on ovarian insulin-like growth factor I (IGF-I) and IGF-I receptor gene expression was investigated. The study groups included five follicular phase, placebo-treated controls, and four testosterone- and three dihydrotestosterone (DHT)-treated rhesus monkeys. The treatment period was 5 days. Both testosterone and DHT treatment resulted in significant, 3-4-fold increases in IGF-I mRNA concentration in granulosa, thecal and interstitial compartments. Likewise, both androgens induced significant increases in the amount of IGF-I receptor mRNA, most notably in thecal cells and less markedly in granulosa and interstitium (P < 0.05). These changes in amounts of IGF system mRNA were documented in growing follicles up to the small antral (</=1 mm diameter) stage. Larger follicles were too few in number for significant comparisons. By contrast, amounts of ovarian insulin receptor mRNA were not appreciably altered by androgen treatment. These data suggest that IGF-I and its cognate receptor may mediate androgen-induced ovarian follicular and thecal-interstitial growth.