Pituitary inhibin B, activin B, and follistatin are local regulators of FSH. Activin B is a homodimeric molecule (ßB-ßB), while inhibin B contains an a and a ßB subunit. The regulation of gene expression of a, ßB, and follistatin by local and endocrine hormones were examined in pituitaries from female rats and in perifused pituitary cells by RT-PCR. Ovariectomy (OVX) induced an elevation in the mRNA level of a and ßB subunits and follistatin. Short-term (4h) treatment of pituitary cells with GnRH decreased both the inhibin a and the inhibin/activin ßB subunit mRNA levels, while long-term treatment (20h) with 100nM GnRH stimulated the expression of both subunits. In contrast, the mRNA level of follistatin was elevated after the short-term GnRH treatment. Long-term exposure of pituitary cells to estradiol and inhibin B suppressed the mRNA expression of ßB and had no effect on the expression of a subunit and follistatin. Our results demonstrate that the increased expressions of inhibin/activin subunits and follistatin in the post-ovariectomy period can be induced by the lack of gonadal negative feedback and the consequently developing high GnRH environment of the pituitary. This study reports first that GnRH administered in high dose and for a long period stimulates the gene expression of inhibin/activin subunits and thereby may contribute to the stimulatory effect of OVX on the expression of these genes.