Morphometric studies were conducted on rat developing submandibular gland. Terminal tubule cells from 5 and 15-day-old, and acinar cells from 15 and 30-day-old animals, were respectively studied. Radii of approximately spherical terminal tubule cell nuclei were measured in 0.5μm thick sections; the nuclear volumes were estimated using BACH's method. The volume of nuclei from non-spherical acinar cell was obtained indirectly. The fractions of cellular volume occupied by the nucleus and cytoplasm in both secretory cells were also evaluated and the cytoplasmic volumes thus obtained. Volume density, surface density and surface-to-volume ratio of the rough endoplasmic reticulum, Golgi complex, secretory granules and mitochondria, and derived stereological parameters, were determined. The secretory cells of older rats exhibited a greater cytoplasmic volume and higher total volume and surface area of organelles than cells from younger animals. The net daily accumulation of rough endoplasmic reticulum membrane surface area in terminal tubule and acinar cells was 157.7μm² and 330.4μm², respectively. These values represent a gain of rough endoplasmic reticulum membrane surface of 10.0% and 6.7%, respectively, along the intervals studied.