Female Sprague-Dawley rats were subjected to a ten week training program to determine the influence of intense interval running on the fiber type composition of selected hindlimb muscles; soleus (S), plantaris (P), deep vastus lateralis (DVL), and superficial vastus lateralis (SVL). The muscles of one hindlimb were used for histochemical ATPase analysis to determine the distribution of fiber types and those of the contralateral hindlimb were assayed biochemically for citrate synthase activity (an aerobic marker). Training induced a significant increase in citrate synthase activity in each muscle section. The largest absolute increase occurred in the DVL and the largest relative increase occurred in the SVL. The distribution of fiber types within the S (85% slow-twitch) and SVL (100% fast-twitch) remained unchanged with training. However, significant increases in the percentage of type I (slow-twitch) fibers in both the P (2-fold) and DVL (3-fold) were observed with concomitant decreases in the type II (fast-twitch) population. In addition, training induced significant changes in the fast-twitch subtype populations of the DVL (IIB----IIA). These data suggest exercise-induced fiber type transformations occurring both within the fast-twitch population and between fast-twitch and slow-twitch fibers in certain hindlimb muscles of the rat following a high intensity interval training program.