The effectiveness of l-epinephrine and l-norepinephrine in enhancing ventricular vulnerability has been compared in twelve dogs by obtaining fibrillation thresholds for single test stimuli. Both agents produce similar increases in vulnerability immediately following injection, and similar decreases in vulnerability at longer intervals following drug administration. These changes in vulnerability occur simultaneously with alterations of the same direction in resting excitability, and are dose-related. It is concluded that these agents have qualitatively and quantitatively similar effects on ventricular vulnerability.