The trial involved 42 cows with clinical milk fever. Thirteen heifers and 32 cows without symptoms served as controls. Calcium, oestradiol and oestrone concentrations in serum were determined in blood samples taken within 6 h before or after parturition. Serum oestradiol and serum calcium concentrations correlated negatively. There was no association between oestrone and calcium concentrations. The results suggest that oestradiol plays a role in the aetiology of milk fever.