Cardiomyopathy was produced by isoprenaline injections to healthy and hydrazinophthalazine-treated rats. The elastin content in the heart muscle and the aortic wall as well as the level of serum elastolytic activity were determined. An increase of elastin content in both tissues was found, and its progress was concomitant with the development of isoprenaline injury. It was also found that the elastolytic activity increased on the second day after the last isoprenaline injection, and on the seventh day of the experiment it began to decrease. Changes in hydrazinophthalazine-treated rats after dosing them with isoprenaline had a similar tendency but they were significantly lower than those found in healthy animals receiving isoprenaline only.