The effect of exercise training on the longissimus muscle types was investigated in rats. There were four groups of rats, each exposed to exercise of equal intensity, but of varying duration. A segment of the longissimus muscle between the 2nd and 4th lumbar vertebrae was used for muscle fiber classification with histochemical ATPase classification. With exercise there were no significant changes in the percentage of the muscle comprised of type I muscle fibers. There was, however, significant increase in the percentage of type II A fibers. This paralleled a decrease in type II B fibers. The percentage of FG fiber types decreased significantly in all groups studied. There was a correlation between intensity and duration of exercise and the degree of decrease seen in the latter fiber types. An increase in oxidative muscle capabilities was evidence by the changes observed in the subtypes of class II fibers.