Adult female rats were exercised by daily swimming. All the increase in heart weight induced by the exercise occurred within 14 days and averaged 30%. The half times of the increases in heart weight and total protein content were about 4.5 days, whereas that of cytochrome c, which was used as a mitochondrial marker, was 6.5 days. The total amounts of DNA and of hydroxyproline in the heart, which were used to evaluate the degree of connective tissue hyperplasia, increased only slightly (8% and 10%, respectively). Other animals were subjected to the same swimming program for 21 days. Groups of rats were killed at various time intervals after stopping exercise. Heart weight, total protein content, and total cytochrome c content decreased rapidly initially, with 60% of the total regression of hypertrophy occurring during the first week. Thereafter, heart weight fell more gradually toward the sedentary control value. The hydroxyproline content of the heart, which was increased 10%, did not decrease during the regression of the hypertrophy.