[en] Hormone production was studied in situ by immunocytochemical methods in 20 pituitary adenomas. Special attention was given to 13 adenomas removed from acromegalic patients. Out of them, 6 had mild to moderate hyperprolactinemia. Immunohistochemistry revealed PRL-containing cells without any close relationship with high PRL serum level in 6 patients. Double immunogold staining revealed mammosomatotrope cells characterized by simultaneous presence of GH and PRL in the same granules in 2 patients. Since mammosomatotropes have never been demonstrated in the normal pituitary, our results probably signify gene dysregulation in pituitary cells of those acromegalic patients.