: OBJECTIVES: The studies related to quantitative ultrastructural features of mammalian pinealocytes, especially in comparative aspects, are relatively rare. Quantitative ultrastructural studies in many mammalian species are lacking. Because of the existence of differences in diurnal melatonin profiles in various species it seemed interesting to compare the ultrastructure of pinealocytes in various mammalian species, both laboratory and domestic animals. MATERIAL AND METHODS: The pineal glands of the following mammalian species have been examined: mouse, rat, Syrian hamster, gerbil, sheep, horse, pig and European bison. For each species 4 adult animals (2 males and 2 females) were used. Cross-sectional areas of the pinealocyte and its nucleus and the relative volumes of the following cytoplasmic organelles were analyzed: mitochondria, lysosomes, Golgi apparatus, granular endoplasmic reticulum, and lipid droplets. The relative volumes of membrane-bound bodies (MBB) in the pig pinealocytes, pigment granules in the horse pinealocytes, and calcareous concretions in the gerbil pineal gland were also analyzed. In addition, numerical density of dense-core vesicles and "synaptic" ribbons was estimated. RESULTS: Ultrastructure of pinealocytes in examined species shows many common features, and existing differences are of quantitative nature only. The observed qualitative differences include MBB in the pig, abundance of pigment granules in the horse, and calcareous concretions in the gerbil. Relative volumes or the numbers of some cell structures (mitochondria, Golgi apparatus, lipid droplets, dense-core vesicles and "synaptic" ribbons) show distinct interspecie differences, whereas those of other cell structures (granular endoplasmic reticulum, lysosomes) are similar in all studied species. CONCLUSIONS: The ultrastructural features of pinealocytes in all examined species confirm earlier suggestions of high activity of these cells. No distinct correlation has been found between the type of melatonin secretion and ultrastructural patterns of pinealocytes. It should be stressed, however, that the ultrastructure of pinealocytes in the present study was studied during the daytime, whereas differences in melatonin secretion are observed at night.