Male Wistar rats were standardized on a "light-dark" cycle. Daily light exposure was maintained from 07.00 to 19.00, while food and water were freely provided to each animal. During one 24-hour period, subgroups of five animals were sacrificed at four-hour intervals. The parotid glands were quickly removed from each animal, after which individual gland volume and weight were determined. Tissue samples were then fixed in glutaraldehyde and osmium and embedded in epon. Other samples were fixed in Carnoy and embedded in paraffin. A stereologic model of the rat parotid gland was constructed by morphometric methods. It could be demonstrated that the volume and weight of the glandular components changed during a 24-hour period. The maximum and minimum for each component were found to vary significantly; in most instances, the maximum occured during the evening at the onset of the eating cycle, whereas the minimum occurred in the morning with the completion of eating. The most dramatic differences were seen within the glandular acinar cells (Fig. 1 and 2). During a normal 24-hour period or physiological secretory cycle, the following volume changes were found: secretory granules, 160%; nuclei, 30%; nucleoli, 80%; golgi-areas, 60%; rough endoplasmic reticulum, 10%. The present study indicates that the parotid gland has a time-dependent structure which reflects the rhythmic activity of the gland in production, storage and extrusion of secretory material.