A total of 60 blood samples obtained from both fertile and infertile cows and buffaloes were analysed for serum calcium, inorganic phosphorus, magnesium. cupper and iron. The results showed that, the serum calcium, inorganic phosphorus and cupper were higher during the oestrogenic phase thn during the luteal phase. The values obtained for the cycling animals were signinficantly higher than those obtained for animals with inactive ovaries. Regarding the serum magnesium and iron there was no differences.