The pancreatic acinar cells of rat embryos obtained at 15, 17, 19 and 21 days of gestation have been examined using fine-structural and morphometric techniques. Morphometric analysis demonstrates significant variations in the average volume of the cell, nucleus and cytoplasm, and the volume, surface and numerical densities of various cytoplasmic organelles during fetal life. In particular, the volume and surface densities of rER exhibit maximal values at 19 days of gestation, suggesting that secretory proteins are produced most actively at this time. Furthermore, membrane continuity between the nuclear envelope and rER is frequently discernible in acinar cells, indicating that at this stage the rER is mainly derived from the nuclear envelope. Zymogen granules first appear at 17 days of gestation. By 21 days, they occupy the greater part of the cytoplasm of the acinar cells, no polarity being seen in their distribution pattern. No direct evidence for the secretion of zymogen granules has been observed during fetal life. It therefore appears that membrane transport involved with intracellular movement of newly synthesized proteins from rER via the Golgi complex to zymogen granules occurs in one direction and lacks regulation.