We first detected the mRNA expression of follistatin and three subunits of inhibin/activin in rat pancreatic islets by reverse transcription-polymerase chain reaction (RT-PCR). Immunohistochemistry using anti-follistatin serum (against residues 123-134) revealed that follistatin was localized only in insulin-producing B cells. Although the beta A subunit was detectable in the islets, the immunostainable cell types were completely different with two beta A antisera, i.e. anti-beta A (1-10)-Tyr stained B cells, while anti-beta A (87-99) stained glucagon-producing A cells. This inconsistent immunoreactivity was probably related to follistatin binding to beta subunits of inhibin/activin. This study indicates that follistatin and inhibin/activin in the islet serve as paracrine or autocrine modulators in the endocrine pancreas.