The mucus secretory granules in the chief cells of the gallbladder epithelium have been studied. The volume density of the mucus secretory granules were 0.6 +/- 0.2%, 1.5 +/- 0.4% and 1.1 +/- 0.4% in normal fasting rabbits, mice and guinea pigs respectively. Electron microscopic observations suggested that these granules were released from the epithelial cells by a mechanism analogous to a merocrine secretion.