Ventricular fibrillation thresholds were measured in anesthetized, closed-chest dogs during the injection of Renografin 76 or Conray 400 into the left coronary artery. Both agents significantly lowered fibrillation threshold-Renografin to 58 .+-. 7%; Conray to 20 .+-. 5% of control, respectively. The experimental method estimates fibrillatory potential of contrast media more precisely than previous methods. The initial results parallel relative toxicities observed in clinical usage.