: OBJECTIVES. Relatively few ultrastructural studies of the pineal gland of aging animals have been published. The pineal gland of the gerbil is especially interesting in respect to aging because of its progressive calcification with age, and this species has been considered as an excellent model for research on aging. Therefore, the aim of the present study was to examine the quantitative ultrastructure of pinealocytes of the gerbil in three different age groups. METHODS. Three groups of animals were included in the study: 1-month-old, 3-month-old, and 14-month-old. Cross-sectional areas of the pinealocyte and its nucleus and relative volumes of the following cell organelles: mitochondria, lysosomes, Golgi apparatus, granular endoplasmic reticulum and calcareous concretions as well as the number of dense-core vesicles and "synaptic" ribbons were analyzed. RESULTS. No age-dependent changes were observed in the size of pinealocytes and their nuclei. The relative volume of mitochondria and the number of dense-core vesicles increased progressively with age, and that of lysosomes was lowest in the 1-month-old animals and increased at age of 3 and 14 months, whereas a decrease in the relative volume of granular endoplasmic reticulum was observed in 3- and 14-month-old gerbils in comparison with 1-month-old animals. No difference was observed in relative volume of Golgi apparatus and in the number of "synaptic" ribbons. The most striking change was observed in the formation of calcareous concretion within the pineal with age. The pineal gland of 1-month-old gerbils was essentially devoid of these structures, their number and size in 3-month-old animals were moderate, and increased dramatically in 14-month-old animals. CONCLUSION. The ultrastructural features of the gerbil pinealocyte in all examined age groups point to high metabolic activity of these cells.