Complementary pharmacological therapy using plant-based raw materials is necessary for normalizing the overall condition and reducing the risk of diabetic complications in type 1 diabetes. The search for natural remedies that prevent dysfunctions, particularly in blood cells, which are primarily exposed to prolonged hyperglycemia impacts, is especially important. This study aimed at evaluating diagnostically significant erythrocyte parameters in animals with streptozotocin-induced diabetes after administration of fruit extracts from Cornus mas cultivars `Uholok´ and `Koralovyi´ and Cornus mas × Cornus officinalis hybrids, for 14 days at a dose of 20 mg/kg body weight per day. The extracts were found to normalise erythrocyte parameters, including reticulocyte content in the blood, daily production of reticulocytes, erythrocyte count, haemoglobin concentrations, and mean cell haemoglobin. Oral administration of the studied extracts led to a rightward shift in the oxyhaemoglobin dissociation curves and an increase in P₅₀ values, indicating a reduction in haemoglobin affinity for oxygen to physiological levels, and caused a decrease in methaemoglobin content. Metabolites of anaerobic glycolysis, antioxidant enzyme activity, oxidative and nitrative stress biomarkers in the erythrocytes of diabetic rats decreased to control levels. The reduction in oxidative damage to erythrocytes, attributed to the strong antioxidant properties of the extracts, contributed to enhanced resistance to haemolysis and an increase in sialic acid-containing glycans that provide erythrocytes with a negative charge, reducing their aggregability. Cornelian cherry fruit extracts are promising sources of bioactive compounds for the development of potential antidiabetic medicines, while erythrocytes serve as indicators of the effectiveness of such pharmacotherapy.