The relationship between the blood constituents of beef and dairy cattle and their age was investigated. The experimental group comprised 12 Afrikaner cows and their calves and 12 Friesian cows and their calves. Blood samples were drawn every fortnight and analysed for 12 constituents. Age influenced the composition of blood significantly. Creatinine, globulin, haemoglobin, erythrocytes, N.P.N., glucose, alkaline phosphatase and inorganic phosphorus in blood differed significantly between young, mature and old cows. The blood of calves in both breeds exhibited significantly higher values than the blood of their dams for erythrocytes, N.P.N., glucose, alkaline phosphatase and inorganic phosphorus. The blood of the Friesian calves also showed significantly higher values over that of their dams for haemoglobin, potassium and sodium.