Nelson's syndrome develops in 10-15% of patients with Cushing's disease who undergo bilateral adrenalectomy. Whether the pituitary tumors of Nelson's syndrome are autonomous or are regulated by hypothalamic signals or glucocorticoids is controversial. We, therefore, compared the plasma ACTH responses to synthetic ovine corticotropin-releasing hormone (CRH) in 11 patients with Nelson's syndrome, 1 patient with Cushing's disease who had had bilateral adrenalectomy and did not have Nelson's syndrome, 14 patients with Cushing's disease, and 27 normal subjects. The plasma ACTH response to CRH in 10 patients with Nelson's syndrome was markedly increased and prolonged compared to the responses of normal subjects or patients with Cushing's disease. In 4 patients with Nelson's syndrome, plasma ACTH and cortisol concentrations also were determined at frequent intervals for 10-24 h during continuous infusions of 0.15 M saline or CRH (1 microgram/kg X h). There was no desensitization of ACTH secretion during short term continuous infusion of CRH. Exogenous cortisol inhibited CRH-stimulated ACTH secretion. These findings suggest that the ACTH response to CRH of patients with ACTH-secreting tumors of Nelson's syndrome differs from the response of those who have the microadenomas of Cushing's disease in two ways: the magnitude is greater, and the response is prolonged. These differences can be explained by the greater size of the tumor and the reduced glucocorticoid feedback in adrenalectomized patients with Nelson's syndrome.