The fine structure of the ergastoplasm of the pancreatic exocrine cell of Swiss albino mice has been studied with the electron microscope. It was found that this material consists of sac-like structures, which may be called ergastoplasmic sacs, embedded in an amorphous granular ground substance, the cytoplasmic matrix. The membranous wall of the ergastoplasmic sac is a structure approximately 250 A wide. Except for its greater electron density and granular structure, the ergastoplasmic membrane is similar in appearance to the nuclear, plasma, and mitochondrial membranes. From data available in the literature, and from our own evidence, the conclusion can be drawn that the ergastoplasmic membrane contains ribonucleic acid. The mode of formation of the ergastoplasm and secretory granules was studied in animals which were first fasted and subsequently fed. It was found that ergastoplasm is formed within the cytoplasm, near the nuclear membrane, and possibly from the plasma membrane. The secretory granules were observed to arise by accumulation of materials within small ergastoplasmic sacs.