Many effects that oestrogens and progestrogens used in oral contraceptive (OC) have on enzyme physiology are of importance on homeostasis. This study was carried out in order to determine the in vivo effect of three oral contraceptives containing ethinyl estradiol in combination with desogestrel and levonorgestrel on the paraoxonase (PON), catalase (CAT) and carbonic anhydrase (CA) activities in mice, which are model organisms for humans. Serum and liver paraoxonase activities were determined spectrophotometrically by using paraoxan as a substrate according to the methods of Gan et al. and Gil et al., respectively. Catalase and carbonic anhydrase activities were determined from erythrocytes used Aebi and Maren methods, respectively. For these studies, a group of ten mice (25±2 g) was selected for oral administration for 21 d of each drug (0.15 mg desogestrel+0.03 mg ethinylestradiol (A); 0.15 mg levanogestrel+0.03 mg ethinylestradiol (B) and 0.15 mg desogestrel+0.02 mg ethinylestradiol (C)). A group of ten mice was included in the study for a control group, which were not subject to drug administration. For each drug, a mean of the serum and liver paraxonase activity and erythrocytes catalase and carbonic anhydrase activities were determined and compared to the control groups. While mouse liver PON activity showed a statistically significant decrease for all three drugs, serum PON activity increased. Erythrocytes catalase activity was significantly decreased by all contraceptives used. On the other hand, these contraceptives did not change the erythrocytes carbonic anhydrase activity.