Morphometric analysis demonstrated a twofold increase in the surface area of the hepatic endoplasmic reticulum in Fischer 344 rats between 1 and 20 months of age, followed by a significant decrease in this parameter between 20 and 30 months. These changes are attributed to the smooth-surfaced endoplasmic reticulum, since neither the rough-surfaced variety nor the Golgi membranes underwent any significant change in surface area as a function of the age of the animal.