A patient with diabetes mellitus treated with the sulfonylurea chlorpropamide developed the syndrome of inappropriate secretion of antidiuretic hormone. The mode of clinical presentation was one suggesting a central nervous system disorder. A clinical study of the patient’s response to chlorpropamide was carried out Headache, systolic hypertension, serum hypo-osmolality, hyponatremia, urine hyperosmolality, fall in urinary output and continued renal excretion of sodium were demonstrated, confirming the initial diagnosis.