In 30 patients undergoing coronary angiography the effects of Iopamidol on sinus node activity, atrioventricular and intraventricular conduction were evaluated using His Bundle recordings, and compared to the effects produced by Diatrizoate. Diatrizoate resulted in an increase in sinus cycle length and prolonged AV nodal conduction, resulting in second degree AV block in two patients. Iopamidol had significantly less effect on sinus cycle (P less than 0.01) and AV nodal conduction (P less than 0.01). Neither contrast media affected intraventricular conduction. No difference was noted in the imaging power of the two contrast media. It is suggested that non-ionic, low osmotic contrast media such as Iopamidol will result in increased patient safety during coronary angiography without loss of imaging power.