To clarify the site and mode of action of tumor necrosis factor (TNF) in the pituitary, we studied the effects, binding sites of TNF and its receptor mRNA in the two types of mouse pituitary-derived cell lines, AtT-20, ACTH-producing cells and TtT/GF, folliculo-stellate (FS)-like cells. First, we examined the expression of TNF receptor mRNA in these cells. Using Northern blot analyses with radiolabeled cDNA to murine TNF receptor p60 and p80 mRNAs as probes, we identified both types of mRNA in the poly(A)-containing RNA prepared from AtT-20 cells and p60 TNF receptor mRNA from TtT/GF. The identified mRNA were compatible in size with those detected in the immune-competent cells. Next, we studied the TNF-binding sites on these cells. Scatchard plot analysis of the significant binding of [125I]TNF revealed a single type of binding site with a Kd (dissociation constant) of 210 pM and 131 binding sites/cell on AtT-20. Similarly on TtT/GF, [125I]TNF showed 353 binding sites/cell with a Kd of 900 pM. [125I]TNF binding on both types of cells competed with TNF and lymphotoxin (TNF beta) in an equimolar fashion. Third, TNF stimulates ACTH synthesis in AtT-20 cells, while TNF increases immunoreactive interleukin (IL)-6 release from TtT/GF cells. These findings demonstrate that AtT-20 and TtT/GF cells are equipped with fully functional TNF receptor system, and suggest that ligand of the receptor, TNF alpha and/or TNF beta, can modulate ACTH synthesis and release as a direct hormonal effector on corticotrophs or indirect modulator through another paracrine mediator, such as IL-6 from FS cells.