We evaluated the usefulness of a desmopressin (DDAVP) test in the diagnosis of ACTH- dependent Cushing's syndrome. After an intravenous injection of 5μg DDAVP, plasma ACTH levels increased to more than 200% of the basal levels in 10 of 10 patients with Cushing's disease, but remained less than 150% in all of 11 normal subjects, 3 patients with Addison's disease, 5 cases of Cushing's disease in remission, and 3 patients with ectopic ACTH syndrome. Peak levels of plasma cortisol after the DDAVP stimulation were 159±14% in the patients with Cushing's disease, and less than 150% of the basal levels in the other 5 groups. We also found a case of Cushing's disease with periodicity which responded to DDAVP only in the active stage. In vitro studies revealed that DDAVP directly stimulates ACTH release from corticotropic adenoma cells through V1b but not V2 vasopressin receptors. In conclusion, the DDAVP stimulation test, i.e., determination of plasma ACTH levels after 55μg DDAVP injection, seems useful for discriminating Cushing's disease from normality, and may serve to facilitate the differentiation between Cushing's disease and ectopic ACTH syndrome.