Magnesium is directly involved in the regulation of exo- and endocrine function of the pancreas. However, the efficacy of different magnesium agents in activating the pancreas function is uncertain. One possible reason for that is the age differences in the structural and functional response of the pancreas to magnesium administration. The aim of this study was to investigate the age-related patterns of changes in the morphofunctional state of the pancreas following prolonged magnesium chloride administration. The experiments were performed on 48 male Wistar rats aged 3 and 15 months. The animals in the experimental group received magnesium chloride (50 mg/kg body weight) in addition to the normal feeding ration. At the end of the experiment the histo-morphological evidence of lowered functional activity of both exocrine (reduced size of the acini and the exocrinocytes) and endocrine (reduced relative area, number and size of the Langerhans islets) was observed in 3 month old rats. In contrast, the administration of magnesium chloride to 15 month old rats moderately increased the activity of the exocrine part of the pancreas (increased size of the exocrinocytes). The significant increase of the relative exocrine part area, linear size of the pancreatic islets, number and density of the endocrinocytes in the islets, suggests that the endocrine activity of the pancreas is increased in adult rats. The tendency for the increased relative stroma area and stromal-parenchymal index was observed in 3 month old rats in the experimental group. These values were decreased in 15 month old rats. The results of the study indicate the age-related differences in the changes for exocrine and endocrine parts of the pancreas following the prolonged magnesium chloride administration. The nature and prominence of these changes suggest that the magnesium lowered the activity of the exo- and endocrine parts of the pancreas in 3 month old rats, and increased it in 15 month old animals.