The natural magnetic field influences the physiological and pathological processes of the body. The authors have followed the modifications of the reactive oxygen species and of the antioxidants in the serum of pregnant rats exposed to zero magnetic field. White pregnant Wistar rats, 200-250 g weight, were divided into two groups, with 9 animals in each group: group I, control pregnant rats 2 weeks in natural magnetic field; group II, pregnant rats exposed to zero magnetic field for 2 weeks. All animals were humanely killed at the end of the experiments. The exposure was made at the National Institute of R&D for Isotopic and Molecular Technology. Rats housed in wooden cages were placed into a device that compensated the natural magnetic field. The device consisted of Helmholtz coils with a 1.2 m diameter and oriented in a North-South direction and along the local magnetic field lines. The field inside the coils was in the range of 200-400 nTesla and it was conventionally named ‘zero magnetic field’. The serum determinations were made in the Laboratory of the Physiology Department U.M.Ph ‘Iuliu Hatieganu’ Cluj-Napoca. Blood samples were collected through venous puncture of all rats. Serumal ceruloplasmine was measured using the Ravin method (1), carbonylated proteins through the Reznick method (2) and malondialdehyde (MDA) by the colorimetric method with TBA (thiobarbituric acid). The results obtained were compared against each other and also to the control group. The serum ceruloplasmine presented increased values in group II (zero magnetic field): 41.39 ± 6.61 mg % compared to the control group I: 26.56 ± 3.2 mg %. The values of the carbonylated proteins were lower in group II: 1.28 ± 0.059 mmol/ml than the contol group I: 1.87 ± 0.42 mmol/ml. Malondialdehyde presented decreased values in group II: 1.29 ± 0.12 mmol/ml in comparison with the control group I: 2.28 ± 0.17 mmol/ml. The animals exposed to zero magnetic field presented significantly decreased values of the malondialdehyde and of the carbonylated proteins, probably because of some lesions in the lipid and protein strata and the serum ceruloplasmine presented increased values in this group (II).