The responses of the adenyl cyclase of human endocrine tumors to hormone stimulation have been examined. Among three parathyroid adenomas, two chief cell adenomas were stimulated by glucagon while an oxyphil adenoma was not stimulated by any hormone tested. A thyroid follicular carcinoma was stimulated by TSH. The cyclase of a chromophobe adenoma was responsive to ACTH. The pheochromocytomas from two patients were not stimulated by any hormone tested. In contrast each of the bilateral pheochromocytomas from a third patient was stimulated by glucagon as well as by ACTH and TSH. The results indicate that the adenyl cyclase of some, but not all, human endocrine tumors is susceptible to hormone stimulation. The tumor responses may resemble those of the parent gland, or may be inappropriate for the tissue of origin. Although it is generally recognized that some thyroid neoplasms are influenced by TSH, the present experiments raise the possibility that other endocrine tumors usually considered to be autonomous may also be under hormone influence, sometimes of an unexpected and inappropriate type.