Cardiac arrest has a very poor prognosis, especially with prolonged efforts at resuscitation, and unfortunately, survivors are often severely neurologically impaired. CPA in children is often the result of a prolonged illness rather than a sudden, primary cardiac event as is frequent in adults. This necessitates that resuscitation research must be conducted separately for pediatric and adult patients. Authorities currently endorse the use of epinephrine for restoring spontaneous circulation based on its ability to maintain diastolic blood pressure and subsequent blood flow to the heart during resuscitation. However, human studies have shown no clear survival benefit of epinephrine and have elucidated concerning adverse effects. Recently, both the European Resuscitation Council and the American Heart Association have recognized the use of vasopressin as a promising vasoconstrictor and an alternative or adjunct to epinephrine in the resuscitation of adults. Vasopressin causes profound vasoconstriction without the adverse effects of epinephrine and is associated with improved blood flow to the heart and brain. This increased cerebral blood flow has been associated with better neurologic outcome in animal studies. In light of compelling animal and human studies of combined vasopressin and epinephrine, pediatric trials are indicated for vasopressin usage in pediatric CPR. This study will evaluate the addition of the administration of vasopressin to standard advanced CPR therapy (epinephrine alone) for pediatric patients that experience in-intensive care unit CPA to assess for improved time to return of spontaneous circulation (ROSC), survival to 24 hours, survival to hospital discharge, and neurologic outcome. When a patient experiences a CPA, standard Pediatric Advanced Life Saving (PALS) protocols as endorsed by the American Heart Association will be initiated. This will include receiving epinephrine as the first vasopressor medication. Patients will then be randomized to receive vasopressin (treatment group) or epinephrine (control group) as the second vasopressor medication, if needed. If more then two doses of vasopressor medication is required in either group, epinephrine will be administered according to the PALS algorithm until the end of the event. All CPA events meeting inclusion criteria will be entered into the National Registry of Cardiopulmonary Resuscitation (NRCPR) Database, which tracts all CPA events at Children's Medical Center Dallas. Prior to commencement of the RCT, a pilot trial of 10 patients will be completed to assess preliminary safety, feasibility, and effectiveness of combination epinephrine-vasopressin for pediatric in-intensive care unit CPA refractory to initial epinephrine dosing. All pilot patients will receive vasopressin as the second vasopressor medication.