Pro-opiomelanocortin (POMC) mRNA was detected on paraffin sections by in situ hybridization (ISH) in corticotrophs of 12 nontumorous pituitaries, 11 functioning corticotroph, and 11 silent pituitary adenomas. ISH combined with immunocytochemistry for adrenocorticotrophic hormone (ACTH), a POMC-derived peptide, was also performed. ACTH immunoreactive cells of the anterior lobes and those invading the posterior lobe showed a high or moderate level of POMC mRNA that was not correlated with the intensity of ACTH immunoreactivity. Variable levels of POMC gene expression were present in Crooke's cells, corticotrophs suppressed by glucocorticoid excess. Most functioning corticotroph adenomas and silent subtype 1 adenomas had an intense hybridization signal and ACTH immunoreactivity. In silent subtype 2 and 3 adenomas, POMC mRNA had a diffuse low level or was absent; in these adenomas ACTH immunoreactivity was diffuse, restricted to some cells, or negative. The results indicate that POMC gene is expressed in both normal and suppressed nontumorous corticotrophs. Intense signals for POMC mRNA are found in most functioning corticotroph adenomas. The difference between POMC gene expression in silent 1 and silent 2 and 3 adenomas suggests that different mechanisms are responsible for the lack of endocrine activity.