Two patients with the syndrome of inappropriate secretion of antidiuretic hormone were studied in a metabolic ward during treatment with 1.2 g demeclocycline daily. In both patients, demeclocycline treatment led to increased renal water excretion with consequent correction of hyponatremia and hypo-osmolality. Three episodes of reversible deterioration in glomerular filtration rate developed in these patients. Each episode was accompanied by clinical evidence of extracellular fluid volume contraction, and on each occasion there was an inappropriate natriuresis with daily urinary sodium excretion remaining above 50 mEq. Although demeclocycline effectively reverses the electrolyte abnormalities of this syndrome, the potentially dangerous side effects that may develop exclude the routine usage of the drug.