Morphological changes of the pancreas of young rats after exposure of exogenous melatonin in the spring and autumn periods was investigated. Exogenous melatonin (Unipharm Inc., USA) was administered to experimental group of animals daily at a dose 5 mg/kg. The duration of the experiment was 28 days. It was revealed that the exocrine part of the pancreas responds differently to the effects of melatonin at different times of the year. Thus, after administration of melatonin in the spring increase of the size of acinus, the height of the epithelium (by 7 %), area exocrinocytes (by 58 %), of their nucleus (by 20 %) and cytoplasm (69 %), the amount of nucleoli in cells (18 %), reduction the amount of connective tissue elements. Melatonin introduction in the autumn decrease in the size of acinus, height and area of exocrinocytes, growth the number of exocrinocytes in the acinus, nucleoli and width layers interlobular connective tissue in the gland. This may indicate that melatonin increases in the spring of the synthetic activity of the exocrine pancreas, whereas in the autumn (for the majority of the morphometric parameters) – somewhat reduces its functional state. The administration of melatonin in the spring (mostly) and in the autumn periods increased the functional activity of the endocrine pancreas. This is indicated by growth in the size of Langerhans islets, increasing the number and density of the (autumn) endocrinocytes.