The current treatment of choice for hyponatremia is water restriction. This therapy is effective but results in a poor quality of life due to significant stimulation of thirst. This study is intended to demonstrate the ability of a daily oral agent to correct a patient's hyponatremia and maintain the patient at normonatremia. The primary objective of this study is to assess the efficacy of a fixed, maintenance dose of OPC-41061 in patients whose hyponatremia has been corrected following the titratioin phase of the study, and to assess the safety of OPC-41061 in each phase of the study. The secondary objectives of the study are to assess the ability of OPC-41061 to induce a normal serum sodium concentration in hyponatremic patients and to determine dose-response characteristics of OPC-41061 on serum sodium concentration. OPC-41061, a benzazepine derivative, is a specific and selective, nonpeptide antagonist of the vasopressin V2 renal subtype receptor, developed by Otsuka Pharmaceutical Company. The study is multi-center, randomized, open-label, active-controlled, dose titration study of a non-peptide vasopressin V2-receptor antagonist, OPC-41061, for the treatment of hyponatremia in euvolemic or hypervolemic states. Patients will be hospitalized in the GCRC on Study Day 0 through Study Day 14 and may be discharged after evaluation on Study Day 14. The volunteers will report to the GCRC unit and will be randomized at Day 0 between the active and the placebo groups, but all patients will receive placebo on study days 0 and 1. On days 0 and 1, serum sodium concentration will be measured. Each subject will be given a no added salt standardized diet from the GCRC kitchen and fluid intake and output will be strictly monitored and recorded. Activity will be restricted to light activity for the first 4 hours after placebo/study drug administration. Patients should not walk or stand for more than a total of 45 minutes during this 4 hour period. Water intake will be limited to a maximum of 1000 ml/day. Urine measurements and output will be recorded. Titration Inpatient Phase (study days 2 to <14): On days 2-14, subjects will receive a single tablet of Study Drug or placebo with 200 cc of water. Patient will be fasting from food only, from midnight the night before on all study days. No meal will be served until 4 hours post-dose on Study Days 2-14. The lowest dose of OPC-41061 shall be given first and will continue on subsequent days using incrementally higher doses. Sodium measurements, vital signs, laboratory tests and ECG will be performed daily. Fluid intake and output monitoring will be strictly maintained. Outpatient Phase (study days <15 to 28): and Follow-Up Phase (study days 35 and 65): The patient will continue to receive daily doses of OPC-41061 (or fluid restriction and placebo) at the dose (or volume) administered at discharge. After discharge, every other day, vital signs, temperature and weight, laboratory tests, and ECG will be performed. Follow-Up visits will occur on Days 35 and 65 when vital signs, laboratory tests, ECG and thirst scale will be performed. Due to slow accrural at all sites, this projected has been terminated by the sponsor.