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Calcium was added to diatrizoate (Renografin 76) and metrizamide (Amipaque) to concentrations from 2.5 to 50 mEq/I. The solutions were injected into the left coronary artery in dogs with either the chest opened or closed. In open chest experiments contractile force and aortic pressures were recorded. In closed chest dogs, left ventricular pressure, first derivative of that pressure and aortic pressure were registered. The calcium ion concentration was linearly, inversely related to the effect of the contrast solutions on the recorded parameters. In these experiments the optimal amount of calcium is suggested to 40 mEq/I added to the diatrizoate solution but only 10 mEq/I added to the metrizamide solution. No ECG changes due to calcium addition could be registered. The cardiac toxicity of these contrast media seem to be due to an interference with the calcium flux across the cell membrane. The effect on contractility can be recorded with the same reliability in either closed or open chest experiments.