Hyponatremia is a frequently encountered electrolyte disorder which can produce severe neurological complications. Excretion of free water by the kidney is often impaired. In many instances, this results from increased levels of circulating arginine vasopressin (AVP), an antidiuretic hormone (ADH).The purpose of this protocol is to assess the efficacy, safety, and dose characteristcs of titrated doses of OPC-41061, a vasopressin V-2 receptor antagonist in hospitalized patients with hyponatremia. Recruitment continues.