The feeding regime and physical activity of horses are modified according to day length and weather conditions, which vary between individual seasons of the year. These changes may be reflected in peripheral blood parameters, key indicators of equine health and welfare. This study aimed to determine the effect of season on changes in haematological and biochemical blood parameters in cold-blooded mares. The experiment involved 22 coldblooded mares aged 9 ± 3 years. Blood samples were collected from each mare to analyse haematological parameters, i.e., haematocrit (Ht), haemoglobin (HB), red blood cells (RBC), white blood cells (WBC) with a leukogram, and biochemical parameters, including glucose, total protein, urea, creatinine, total cholesterol (TCH), triacylglycerols (TG), high-density lipoprotein cholesterol (HDL), low-density lipoprotein cholesterol (LDL), alkaline phosphatase (AP), alanine aminotransferase (ALT), aspartate aminotransferase (AST), lactate dehydrogenase (LDH) and concentrations of P, Ca, Mg, Cu, Fe, and Zn. Blood was sampled in February (winter), May (spring), August (summer), and November (autumn). The analyses demonstrated that the values of most blood parameters tested varied between seasons, except for RBC and WBC counts, and urea level. The lowest Ht and HB levels, as well as the lowest percentage of granulocytes, were recorded in spring, whereas several biochemical markers, including total cholesterol, TG and HDL had lower values in autumn. Therefore, seasonal variations of the analysed blood parameters should be considered when diagnosing equine health and disease.