The aim of the study was to verify the hypothesis that brassinosteroids would modify the cold acclimation process through the stimulation of photosynthesis and carbohydrate metabolism, and would consequently cause an increase in the plant’s capacity for winter survival. Seedlings of two winter rye cultivars (winter-resistant and moderately winter-resistant types) were sprayed with 24-epibrassinolide (EBR) and then subjected to cold acclimation at 4 °C. The foliar hormone application was studied for its effects on two components of winter resistance: tolerance to frost; and resistance to snow mould. These were examined after medium (3 weeks) and long (6 weeks) terms of cold acclimation. Additionally, as a control group, the level of endogenous brassinosteroids was measured in non-treated plants. The maximum acquired snow mould resistance was observed in the EBR-treated plants after 3 weeks of cold acclimation, while the maximum frost tolerance was recorded after 6 weeks of cold acclimation, as compared with control. In the winter-resistant cultivars, the EBR-induced snow mould resistance coincided with a lower photosynthetic efficiency and a lack of change in their Rubisco activity and level of total soluble carbohydrates. The elevated EBR-induced frost tolerance in the winter-resistant cultivars was associated with a carbohydrate metabolism that promoted a decrease in the monosaccharides of the leaves that stimulate photosynthetic efficiency and Rubisco activity. For both cultivars, the control plants developed frost tolerance between the third and sixth week of cold acclimation. However, the winter-resistant cultivar was characterised by a constant castasterone (CS) level, while the moderately winter-resistant one experienced a sharp increase in its endogenous CS after 3 weeks, and then after 6 weeks of cold acclimation.