Activin B (beta B beta B) may function as an autocrine modulator of basal FSH secretion and expression based on the previous observation that beta B is expressed within gonadotropes. The incubation of cultured rat anterior pituitary cells with a m mouse monoclonal antibody specific for the activin B homodimer (MAb-activin B) significantly attenuated the basal secretion of FSH in a concentration- and time-dependent manner, without influencing LH secretion. Moreover, MAb-activin B selectively inhibited FSH beta mRNA accumulation without affecting either LH beta or alpha subunit mRNAs. The secretion of activin B by the gonadotropes of the anterior pituitary may serve as an autocrine signal in the selective modulation of FSH expression and secretion. Furthermore, the inhibitory actions of inhibins and follistatins on gonadotropes may, in part, be explained by their ability to interfere with the actions of endogenous activin B.