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Metrizamide is a new non-ionic water soluble contrast agent. Isolated rabbit hearts were perfused with solutions of 1) metrizamide, 2) metrizamide with plasma equivalent amounts of cations, 3) oxygenated metrizamide, 4) oxygenated metrizamide with cations, 5) diatrizoate. Solutions of 1) metrizamide, 2) metrizamide with plasmaequivalent amounts of cations, 3) metrizamide with calcium ions, 4) metrizoate (ionic medium) with calcium ions, metrizoate with 5) low and 6) high sodium content and 7) diatrizoate were injected into the left coronary artery of dog hearts in vivo. Maximal change of myocardial contractile force was measured with a strain gauge arch. Changes in aortic blood pressures were also recorded. In both sets of experiments metrizamide affected these parameters less than the ionic contrast media. The addition of calcium ions to both ionic and non-ionic media reduced the adverse effects on the observed cardiac functions, while simultaneous addition of plasma equivalent amounts of four cations as metrizoate salts had no beneficial effects. Sodium ions had in high concentrations deleterious effects on cardiac performance. Oxygen saturation of the contrast medium had no observable effect. The adverse effects on cardiac mechanical function of sodium ions and possibly other cations contained in a contrast medium solution might be counterbalanced by the addition of calcium ions to the solution.