Endothelins modulate hormonal secretion in the pituitary gland. Intense signaling of endothelin A receptors (ETAR) has been detected by in situ hybridization, binding assay and receptor autoradiography. We used light- and electron-microscopic immunohistochemistry of ETAR with polyclonal antibody against a synthetic peptide corresponding to the carboxyl terminus (403-427) of human ETAR. Immunoreactivity was observed in 6-8% of anterior pituitary cells, which were rather large polygonal or stellate cells. These cells were often clustered. Double-staining immunofluorescence showed that. the ETAR-positive cells immunoreacted with antibody against the beta-subunit of thyroid-stimulating hormone (TSH), but not adrenocorticotropic hormone (ACTH) or lutenizing hormone beta (LH beta). Pre- and postembedding electron-microscopic immunohistochemistry showed that ETAR-positive cells had vacuolated or parallel-lined rough endoplasmic reticulum (rER) and numerous round granules in their periphery and the elongated processes. By pre-embedding immunohistochemistry, diaminobenzidine tetrahydrochloride (DAB) products were shown to be mostly located around the granules and occasionally underneath the plasma membrane. By postembedding immunohistochemistry, granules in the ETAR-positive cells were 90-150 nm in diameter, and colloidal gold particles due to ETAR were associated with about 10% of these granules. These results indicate that ETA receptors are associated mostly with the secretory granules of TSH cells.