Coronary angiography with Renografin-76 (meglumine sodium diatrizoate, Squibb) lowers the ventricular fibrillation threshold (VFT) significantly and in a dose-dependent manner. Recently new contrast materials have been synthesized for possible angiographic use. We compared the fibrillatory propensities of Hexabrix (meglumine sodium ioxaglate, Guerbet), a monacid dimer; Exypaque (Nyegaard; U.S. iohexol, Winthrop), a new nonionic agent; and Amipaque (metrizamide, Winthrop), an older nonionic agent, to Renografin-76 in anesthetized dogs. The VFT following subselective left circumflex artery injections of 370-mg I and 740-mg I doses of each agent was compared to the control (no injection) threshold in 115 paired experiments in 25 dogs. At the low dose, the VFT for Renografin was significantly lower (61.8 +/- 2.3% of control threshold) than that of Hexabrix or Exypaque (69.3 +/- 2.1%, respectively; P less than 0.05). At the higher dose, the differences were more pronounced among Renografin, Hexabrix, and Exypaque (32.4 +/- 1.4%, 65.7 +/- 2.6%, and 59.2 +/- 2.4%, respectively; P less than 0.001). No statistical difference was detected between the latter two agents at these doses in nonischemic dogs. Amipaque was also less fibrillatory than Renografin and equivalent to the other two agents. These newer agents appear to offer enhanced safety for use in angiographic studies in the heart.