This is a multicenter worldwide study with 32 participating men and women aimed to figure out whether SR121463B is safe and effective in correcting low levels of sodium in blood in subjects with SIADH. The participation in the study lasts up to 34 days. After screening, during the first part of the study (maximum 5 days), the subjects receive either capsules of SR121463B or a matching placebo. During the second part of the study (23 days), subjects receive either capsules of SR121463B or no treatment. The subjects will be assessed by physical examinations, electrocardiograms, blood samplings, and urine collections. Women of childbearing potential must have an approved method of contraception.