When primary culture of rat pituitary cells were incubated with 1 nM activin-A for more than 24 hrs, activin-A significantly increased intracellular content of FSH without affecting the control of LH. Pretreatment of the cells with activin-A also enhanced LHRH-induced FSH release without affecting LH release. Furthermore, pretreatment of the cells with activin-A significantly reduced both GRF-mediated GH release and TRH-mediated PRL release. However, activin-A did not affect the response of ACTH and TSH to their releasing hormones. These results indicate that, in addition to the known action on gonadotrophs, activin-A also modifies the function of somatotrophs and lactotrophs.