One of the complications of diabetes mellitus (DM) is diabetic myopathy, which is diagnosed in 88% of patients. Therefore, the purpose of our study was to establish the age characteristics of histo-ultrastructural changes in the masticatory muscle of 2-month (immature) and 6-month (mature) rats with streptozotocin-induced diabetes mellitus (STD). It was induced by a single intraperitoneal injection of streptozotocin 7 mg/100 g body weight in 2-month animals, and 6 mg/100 g body weight in 6-month-old rats. It was found that on the 70th day of STD there was a muscle sequestration and focal aseptic inflammatory cell infiltrates in the masticatory muscle of 2-month-old rats, while in 6-month-old animals the fibrosis and focal lysis of individual muscle fibers (MF), sometimes their necrosis and partial replacement by connective tissue were noticed. Sonography revealed in 6-month-old rats partial necrosis of MF, karyorexis, subsarcolemic edema, fibrosis and lysis of myofibrils, destruction of mitochondria; 2-month animals had intracellular regenerative processes: formation of young mitochondria, appearance of numerous granules of glycogen in the sarcoplasm between myofibrils. Such changes occur against the background of the diabetic microangiopathy. Adult animals had a massive destruction of neuromuscular junctions (NMJs) and it leads to a decrease in the area of axon sprouting by 67% and is accompanied by partial destruction of neuromuscular synapses. In 2-month-old rats, there was also a decrease in the area of NMJs by 54%, while in some NMJs the reinnervation processes were noted due to collaterals from neighboring nerve fibers. Thus, diabetic myopathy is diagnosed in the masticatory muscle of SDM rats of different ages, which results from the development of diabetic microangiopathy and peripheral neuropathy. In adult animals, it leads to atrophy and partial sclerosis of MF of the masticatory muscle, and in immature animals, compensatory-restorative processes are observed in the NMJs and MF.