We have recently demonstrated that the rat ovary expresses LHbeta, FSHbeta, and the common alpha subunit mRNA. In the present report, we studied the regulation of LHbeta and of gonadotropin-releasing hormone (GnRH) mRNA expression in granulosa cells that were isolated from immature rats treated with either estrogen or pregnant mare serum gonadotropin (PMSG). In both cell types, GnRH agonist treatment resulted in a decrease in LHbeta mRNA expression. However, only in cells derived from PMSG-treated rats, GnRH treatment produced an increase in GnRH mRNA expression. A markedly increased GnRH mRNA expression was also obtained in granulosa cells derived from PMSG-primed rats in response to LH. In addition, FSH reduced the expression of LHbeta mRNA in granulosa cells from estrogen-primed rats. These results suggest that the expression of LHbeta in the ovary is regulated by locally produced GnRH and by FSH from either the ovary or the pituitary.