During a prolonged incubation (for 3 hr at 37℃ under aerobic conditions), the level of total glycogen phosphorylase and that of glycogen phosphorylase were significantly reduced. The estimated values of total glycogen phosphorylase were 70-60 %, while those of glycogen phosphorylase were 40-25% of the originals determined immediately after sacrifice of the animals. In the case of the experiments, however, the level of glycogen phosphorylase b was unchanged. Therefore, it may be concluded that the reduction of total gycogen phosphorylase activity during the prolonged incubation was caused by decrease in the activity of enzyme a.