GH-releasing peptides are a new class of potent GH secretagogs (GHS) in vivo and in vitro. In normal man GHS also elicit low but definite ACTH and prolactin secretion. Recently it was shown that patients with pituitary corticotrope adenomas respond to GHS with a dramatic rise in ACTH secretion, and it has been suggested that GHS may provide a diagnostic tool to differentiate Cushing's disease from the ectopic ACTH syndrome. GHS actions are mediated by a G protein-coupled receptor recently characterized and cloned in man and rat. In this study we analyzed GHS receptor (GHS-R) expression in various types of pituitary adenoma and in endocrine and non-endocrine lung tumors by RT-PCR. GHS-R transcription was detected in all normal pituitaries and GH-secreting adenomas as expected. The receptor was also transcribed in some prolactin-secreting adenomas and non-functioning adenomas, and, more strikingly, in all 18 ACTH-secreting pituitary adenomas studied. Furthermore, it was frequently expressed in endocrine bronchial tumors, especially carcinoids, whereas it was not found or barely detectable in non-endocrine bronchial tumors. Again ACTH-secreting carcinoids of the lung were all positive for GHS-R expression. These results show that GHS-R transcription is a common feature of endocrine tumors independent of their type and origin.