A patient with the syndrome of inappropriate antidiuretic hormone release (SIADH) following head injury and meningitis was studied during treatment with demeclocycline, a drug known to produce a reversible nephrogenic diabetes insipidus. No changes were observed during six days of demeclocycline 1200 mg/24 hr but urine output increased significantly, with the production of a dilute urine, when the dose was increased to 2400 mg/24 hr. The patient lost weight, and all biochemical features of the syndrome were rapidly corrected despite an unchanged fluid intake and despite the persistence of high plasma levels of ADH. The rise in serum sodium was accompanied by mild sodium retention, as measured by external balance and exchangeable sodium. A complication of treatment was the development of acute renal failure possibly induced by a nephrotoxic effect of high circulating levels of demeclocyline. On stopping demeclocyline renal function returned to normal and, after some delay, SIADH returned, and was still present 9 months after initial presentation. This confirms earlier reports of the efficacy of demeclocycline in SIADH; but the authors advise caution against increasing the dose above 1200 mg/24 hr.