Diurnal changes in tissue glycogen stores were determined in rats in relation to the feed timing of sucrose. Rats were daily meal-fed on a 35% sucrose diet at 20.00-21.00 and a basal diet at 08.00-09.00, or meal-fed on the same diets at the reversed time for 7 weeks. Half of the animals were allowed voluntary wheel-running between 21.00-08.00, but the remaining animals were restricted to exercise for 24 h. At the end of the feeding period, both groups of rats were killed at 4-h intervals. Glycogen stores in liver and soleus muscle of sedentary rats showed sharp increases over 8 h after the sucrose meal regardless of its feed timing. The increases continued for only 4 h in exercised rats given the sucrose diet at the evening meal time. In contrast, glycogen stores in these tissues did not show any noticeable increase after the basal diet regardless of its feed timing. Thus, the feeding of sucrose at the evening meal time seems to be more preferable than at the morning meal time in order to have higher glycogen stores in liver and skeletal muscle during the physically active phase of the day in rats. Diurnal changes of glycogen contents in heart and adipose tissue differed from those in liver and skeletal muscle.