Roles of activin/bone morphogenetic protein (BMP) system in the pathogenesis of human pituitary adenoma remain unknown although these factors stimulate follicle-stimulating hormone (FSH) secretion in the normal pituitary. Here we demonstrated that type-I and -II subunit mRNAs of activin/BMP receptors are expressed in Pit-1-negative FSH-producing (FSH-oma) and nonfunctioning pituitary adenomas (NF-oma). Basal levels of serum FSH standardized by luteinizing hormone (LH) were markedly high in FSH-omas in contrast to NF-omas. However, gonadotropin-releasing hormone (GnRH)-induced increment of FSH standardized by that of LH was not changed in FSH-omas, suggesting that imbalanced FSH secretion by FSH-oma is not attributable to GnRH regardless of the expression of GnRH receptor. Although activin betaA subunit was detected in neither adenoma, the betaB subunit was expressed highly in FSH-omas and, to lesser extent, in NF-omas. As for BMPs, BMP-6 and -7 were detected in NF-omas while BMP-4 and -15 were not detected in either type of adenoma. In the presence of pituitary activin/BMP system, the levels of co-expressing follistatin mRNA in the tumors were reduced in FSH-oma compared with NF-oma, suggesting that endogenous follistatin is involved in FSH overproduction through inhibition of activin/BMP system independently of GnRH.