Vasopressin improves peripheral vascular resistance and volume retention without increasing myocardial oxygen demand, offering a potential advantage over epinephrine for cardiac arrest treatment. While early trials showed promise, larger randomized studies did not confirm a general benefit. However, one notable study indicated that patients with asystole might benefit from vasopressin therapy. Our analysis uses data from a large urban EMS system that implemented vasopressin for asystole in January 2005. We compare the rate of return of spontaneous circulation (ROSC) in patients treated before and after this protocol change.