The research was conducted on 240 clinically healthy Holstein-Friesian cows in various stages of the reproductive cycle (lactation, dry period, and up to 15 days from calving). Sampling was taken during the summer and winter periods. Concentrations of calcium, phosphorus and magnesium in blood plasma were determined by using the appropriate method with the "Beckmann DU-64 UV/VIS" spectrophotometer. Significant differences were determined in the values of concentrations of the examined minerals during various stages of the reproductive cycle but in dependence on the season of sample taking.