Mouse gallbladder epithelial cells were studied with the electron microscope during fasting and refeeding. Morphometric data were obtained from randomly selected epithelial cells of normal starved (12, 24, and 48 hr) and refed (12 hr) mice. Deprivation of food significantly diminishes the volume density of the mucinous secretory granules by about 70% after 48 hr of fasting. Upon refeeding, this secretory granule parameter increases significantly ( 2.5 times). Stereological measurements were also performed on nuclei, mitochondria, and lysosomes, but no major morphometric changes were observed in these organelles. The findings suggest that a basal secretion of mucin granules occur in the mouse gallbladder, irrespective of the animal's nutritional state and that this discharge during starvation exceeds the formation of new granule material. The findings are discussed in relation to effects of fasting and refeeding on other secretory cell systems.