A patient with a syndrome of inappropriate antidiuretic hormone secretion secondary to an undifferentiated bronchogenic carcinoma with distant metastases was treated with demethylchlortetracycline. Up until recently, treatment of this syndrome was based on water restriction and when the plasma sodium concentration became extremely low, hypertonic saline solution administration. Recently it has been demonstrated that the antibiotic demethylchlortetracycline inhibits the action of the antidiuretic hormone on the renal tubules. The drug has been used successfully in five patients with the syndrome of inappropriate antidiuretic hormone secretion. The administration of 900 mg of demethylchlortetracycline per day for 7 days in our patient produced an increase of free water clearance, diuresis, plasma sodium concentration, and plasma osmolarity. Urinary excretion of sodium and urinary osmolarity declined. Furthermore, the neurological symptoms attributed to hyponatremia improved markedly. The patient lost 6 kg during treatment, probably because of negative water balance induced by demethylchlortetracycline. Even though the administration of demethylchlortetracycline did not produce significant decreases in the glomerular filtration rate or renal blood flow in our patient, it is advisable to control the renal function in individuals treated with this drug since it may on occasion determine renal insufficiency.