This is a randomized, open label study to investigate vasopressin versus phenylephrine as a first line pressor in cardiac surgery. All patients \>18 years of age presenting for coronary artery bypass graft (CABG), valve surgery, or combined CABG and valve surgery will be screened for inclusion. Patients with ejection fraction \< 35%, \> moderate pulmonary hypertension, \> mild right ventricular dysfunction, a planned radial arterial graft, or circulatory arrest will be excluded. Patients will be randomized to receive either vasopressin or phenylephrine as the first line vasopressor during the perioperative period to maintain mean arterial blood pressure \>65 mmhg. Primary outcome is acute kidney injury. Secondary outcomes are 30-day mortality, stroke, myocardial infarction, re-operation, sternal infection, atrial fibrillation, intensive care unit length of stay, and total vasopressor hours.