This study was carried out on eighty Holstein dairy cows of four years of age and in their second lactation from Bursa region of Turkey. Twenty of them were in dry period and the rest were in lactation period. The days 48-51(st), 147-161(st) and 180. and over were accepted as first, second and third period, respectively. Blood samples taken from animals into heparinized tubes were analysed for erythrocyte sodium, erythrocyte potassium, whole blood sodium, whole blood potassium and haematocrit. There was no statistically significant effect of lactation periods on examined parameters except haematocrit.