Acinar cells of parotid glands from untreated, pilocarpine-treated, and atropine treated rats were studied, using a potassium pyroantimonate technique and X-ray microanalysis of calcium localization at the ultrastructural level. This was done in order to identify intracellular compartmentalisation of calcium and to elucidate any calcium translocation that might occur during the secretory process. Calcium pyroantimonate complexes were shown to be most prevalent on the plasma membrane of the non-secreting cells. These membranes gave up their calcium complexes during secretion. Conversely, the mitochondrial precipitates increased during secretion indicating an increase in intracellular calcium. The function of calcium in parotid gland secretion and the association of calcium with other cell structures are discussed.