Blood is the main carrier of nutrients and the first indicator of changes occurring in an organism. Hence, the aim of the study was to demonstrate the effect of the day length and the total protein content in the diet for farmed fallow deer (Dama dama) on the selected mineral contents of the plasma and blood haematology. The investigation of farmed fallow deer revealed an increase in the level of red blood cells (RBC) and haemoglobin (HGB) induced by altered light conditions, which may the evidence stimulation of erythropoiesis by an increased concentration of androgens. The level of white blood cells (WBC) changed significantly in the control group, which was kept in the least favourable conditions, as indicated by leukocytosis. In the present study, animals exposed to a changed day length exhibited a significant increase in the plasma concentrations of most minerals (phosphorus - P, magnesium - Mg, copper - Cu, calcium - Ca, iron - Fe) after the winter period in the first year of studies, which indicates accelerated development of antlers in farmed fallow deer. However, there was no equally intensive increase in the plasma mineral content (except for Ca) in the same group of animals in the second year of the study. This probably resulted from the administration of adequate nutrition, which prevented changes in the blood mineral concentration despite the antler growth