: OBJECTIVES: The characteristic feature of the pig pinealocytes is the presence of numerous membrane bounded bodies (MBB), which according to our previous results may be involved in the secretory activity. The present study was undertaken to analyze the effect of continuous darkness and illumination on the ultrastructure of the pig pinealocytes. MATERIAL AND METHODS: The study was performed on three groups of gilts. The first group (control) was kept under a cycle of 14hrs light (500 lx) and 10hrs dark per day. The second group was exposed to continuous illumination (500 lx). The third group was kept under red light with intensity less than 1 lx, which was considered as darkness. The pigs were kept for 14 days under above reported conditions and then slaughtered at 08:00. The point count analysis was used in quantitative studies of pinealocyte substructures. RESULTS: The exposition of pigs to continuous illumination resulted in the decrease in the relative volume of mitochondria and in the numerical density of multivesicular bodies as well as in the increase in the relative volume of MBB in pinealocyte cell bodies. The exposition to continuous darkness led to the increase in the relative volume of mitochondria and the numerical density of dense core vesicles as well as induced some changes in smooth endoplasmic reticulum in pinealocyte cell bodies. CONCLUSIONS: The obtained results point to mitochondria, MBB, multivesicular bodies, dense core vesicles and smooth endoplasmic reticulum as the structures of the pig pinealocyte, which are controlled by environmental light conditions.