Infusion of progressive volumes of meglumine sodium diatrizoate (Renografin 76) into the arteries to the sinoatrial and atrioventricular nodes in anesthetized dogs produced a dose rate-dependent decrease in heart rate and an increase in PR interval. At high dose rates, Renografin 76 caused sinus arrest and complete heart block in 50 and 70% of the animals, respectively. Saline and meglumine solutions (0.939 M) equiosmolar with Renografin 76 produced similar effects. Infusions of metrizamide (equal in I content with Renografin 76), dextrose solution (0.939 M) and autologous blood each had substantially less effect. Contrast media have a direct inhibitory action on sinoatrial automaticity and atrioventricular conductivity, which is dependent on ionic strength, not I content.