The dynamics of the physiologic secretory cycle of the rat pancreatic acinar cells along a 24-h span with an alternating 12 : 12 light to dark cycle have been quantitated by morphometric methods. Significant circadian changes could be detected in cell volume and the volumes and surface areas of secretory granules and several organelles involved in the secretory process. During the dark period, which is the activity and feeding period, there is a decrease in the size of cells and individual zymogen granules as well as in the amount of secretory material. This is paralleled by a drop in the number of ribosomes and Golgi vesicles (after a transient increase) and a loss of cytoplasmic membranes. Most of the cell's zymogen content is refilled during the first hours of the light period, when cellular autophagy reaches maximal values.