In forty patients with normal coronary arteries, the electrocardiographic changes secondary to Urografin 76 and Hypaque 85 injection into both coronary arteries were monitored. Hypaque caused significantly greater prolongation of the PR interval (p less than 0.001), depression of the ST segment (p less than 0.05) and depth of T wave inversion (p less than 0.05). These effects were more noticeable during right coronary artery injection. Both contrast media slowed the heart rate equally during right and left coronary artery injection. In the absence of coronary arterial disease the ECG changes secondary to contrast media injection probably reflect a direct toxic effect. It is possible that premedication with atropine will reduce these effects. Urografin 76 appears the less toxic of the two media, although one case of ventricular fibrillation occurred with each.