A 61 year old woman presented with profound hyponatremia and markedly low serum osmolality. Urine osmolality was greater than the serum osmolality, an abnormality that was corrected by water restriction, suggesting inappropriate ADH secretion. Although there were no physical signs of Cushing's syndrome, her serum potassium level was low and markedly elevated levels of plasma and urine corticosteroids were not altered by the administration of large amounts of dexamethasone, suggesting the ectopic ACTH-MSH syndrome. Plasma levels of immunoreactive ACTH and beta-MSH were elevated. At autopsy, a metastastic oat cell carcinoma of the lung, not detected antemortem by chest roentgenograms and bronchoscopy, was found. Immunoreactive ADH, ACTH and beta-MSH were detected in the primary tumor and in metastases to the liver. beta-MSH was also detected in the spleen, in which metastases were observed. This is the first documented case of the simultaneous production of ADH, ACTH and beta-MSH by neoplastic tissue associated with clinical manifestations of the syndrome of inappropriate ADH secretion and the ectopic ACTH-MSH syndrome.