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Left coronary arteriography and left coronary artery injections of Tyrode's solution and glucose solutions were performed in 12 anesthetized dogs. The contrast medium was found to prolong the conduction time through the AV junctional tissue and through the most distal part of the conduction network as verified by His-bundle electrogram and by surface ECG (lead II). The results indicate that for evaluation of the effects on conductivity of various media, it is most reliable to measure the altered conduction velocity in the AV nodal region. The prolongation of conduction time produced by contrast medium explains heart conduction blocks and may contribute to the genesis of ventricular fibrillation, frequent complications to coronary arteriography. The data suggest that anoxemia does not cause this delayed conduction. Hyperosmolarity contributes to the alterations of the conductivity, but the ionic composition and anion toxicity of the contrasts medium also may be of importance.