This pilot study will investigate the association between levels of the peptide copeptin and response to tolvaptan, a drug that blocks the action of the water retaining hormone vasopressin. The study will enroll stable outpatients with CHF with reduced LVEF (≤45) selected by targeting upper and lower quartile copeptin levels at screening (10 each). The treatment phase of the study will be a prospective, single-arm, open label protocol. All patients will receive active therapy consisting of a single oral dose of 30 mg of tolvaptan with body weight, fluid intake, and urine output monitored in a research unit for 24 hours.