The pineal gland of rats 12–28 months old was studied with light and electron microscopes. All pineal components exhibited regressive changes of different intensity with age. In type I pinealocytes, there was a marked increase in dense bodies as well as the occasional appearance of wide cell profiles full of vesicles. Type II pinealocytes showed nuclear infoldings and cytoplasmic deposits of lipofuscin. Pineal stroma displayed an increase in connective tissue fibers, both collagen and oxytalan, as well as remains of basement membranes and other materials of unknown nature. Calcareous concretions were also found, mostly in the pineal capsule. All regressive changes were more intense with increasing ag