A case is described of a patient with an oat cell carcinoma of the bronchus with moderately elevated levels of plasma corticotrophin (ACTH) and antidiuretic hormone (ADH). Ectopic secretion of ACTH induced severe hypokalaemia and concealed the effects of concomitant ADH secretion on renal function. Normal renal responsiveness was restored following correction of hypokalaemia. The hypokalaemia was associated with evidence of a marked increase in corticosteroid secretion but plasma ACTH concentrations did not show a proportionate elevation. Chromatographic studies on tumour extracts suggest that the presence of a large fraction of high molecular weight ACTH in plasma could explain this discrepancy.