Vasopressin is a peptide hormone that is synthesized in the hypothalamus. The primary stimuli for endogenous physiologic release are hyperosmolality, hypotension, and hypovolemia. It is used in the critical care setting for severe catecholamine-resistant vasodilator shock, in which case it acts as a potent vasoconstrictor. Conditions in which vasopressin has been used include septic shock, postcardiotomy shock, and hemorrhagic shock. There are insufficient and conflicting human and animal data to recommend its use routinely to manage shock from poisoning. Further data are needed to define its risks, benefits, and optimum dose. Increases in arterial pressure should be evident within the first hour. Its serum half-life is less than 15 minutes.