In order to know the pituitary reserves of ACTH, GH, LH, FSH, TSH and prolactin in patients with Cushing's syndrome, the responses of these hormones to hydrocortisone, lysine-8 vasopressin (LVP), insulin-induced hypoglycemia, luteinizing hormon-releasing hormone (LH-RH) and thyrotropin releasing hormone (TRH) were examined before and after treatment. Fourteen patients with Cushing's disease (adrenal hyperplasia), 3 patientswith adrenal adenoma and one patient with adrenal carcinoma were investigated. Before treatment, sufficient response of plasma ACTH to LVP was observed in patients with Cushing's disease, while no response was observed in 3 patients with adrenal adenoma. There was no significant difference in the responses of other pituitary hormones between the patients with Cushing's disease. and the patients with adrenal adenoma. The response of plasma GH to insulin-induced hypoglycemia was impaired in most these patients. The response of plasma TSH to TRH was impaired in 6 of 8 patients tested. The response of plasma LH and FSH to LH-RH were preserved in 6 and 5 of 8 patients, respectively. The response of plasma prolactin to TRH was normal in most patients tested. After treatment, the improvements of the impaired responses of GH, TSH, LH and FSH wereobserved. Therefore, the impaired reserve observed in these patients before treatment seemed to be due to the hypercortisolemia. If the difference of the suppressibility of these pituitary hormones by cortisol may be judged simply from our observation, the orderof the suppressibility is supposed to be ACTH, GH, TSH, LH and FSH, and then prolactin.