The purpose of this study is to investigate the effect of high intensity jump training on the proportion of slow-twitch (ST) and fast-twitch (FT) fibers in skeletal muscle from male Wistar strain rats (6 weeks after birth, n=40). Animals were assigned to one of four groups: a control group (n=10) and three exercise groups: Jump A (JA n=10), Jump B (JB n=10), Jump (JC n=10). Training exercise consisted of successive jumping trials of 20 repetitions (JAgroup), 50 reps. (JB group) and 50 reps. ×2 sets (JC group) on the jump height of about 2.3-fold of their body length (tail excluded). Exercise groups were trained 6 days a week for 10 weeks. After the training period, fiber type proportion of the M. gastrocnemius was histochemically examined. The results were summarized as follows: 1) The body weight in three training groups was significantly smaller than that of the control group. 2) Relative muscle weight (muscle weight/body weight) of the M. gastrocnemius in the JC group was significantly larger than that of the control group. 3) At deep region of M. gastrocnemius in JC group, significant decreases in the percentage of ST fibers were observed with concomitant increases in the FT population. These results suggest that the alteration of ST fibers to FT fibers in rats observed after high intensity jump training may be due to the transformation of ST fibers to FT fibers.