Male mice of dd-strain, at 3 weeks of age were used at the start of the exercise training. The exercise training continued successively throughout the 3 to 7 weeks of age. The trained group was divided groups ; namely, light-, middle- and heavy-trained groups, which were exercised on a treadmill. The treadmill exercise program for light-, middle- and heavy-trained groups consisted of running at speeds of 6 m/min, 10 m/min and running up a 10% grade at a speed of 12 m/min for 10 min 5 times a week, respectively. The untrained group was used as control under normal laboratory condition. The skin and Achilles tendon of the four groups were sampled and their collagen content were examined by means of concentration of hydroxyproline. The collagen content in the skin of growing mice did not recognize significant difference among the four groups. On the other hand, the collagen content in the Achilles tendon of growing mice clearly recognized significant difference between the middle-trained and untrained groups and/or between the middle-trained and light-trained groups. However, middle-exercise training reduced the accumulation of collagen in the Achilles tendon of grow. ing mice.