We propose classification of duodenal endocrine cells of intact rats based on ultrastructural signs of secretory granules and subdivided these cells into 10 basic types. The effect of long-term irradiation in a total dose of 2.5 Gy on ultrastructural organization of duodenal apudocytes was studied. Irradiation induced nonspecific changes of cell organelles in apudocytes. Differences in the ultrastructural disorganization were detected between different types of apudocyte populations and between different types of endocrine cells. Under conditions of adaptation to radiation apudocytes released the secretory product not only through molecular extrusion and exocytosis, but also via degranulation.