Purpose: This study was undertaken to evaluate the effects of magnesium sulfate (MgSO₄) on the contractile activity of the uterus in a pregnant rat model of preeclampsia induced by N-nitro-arginine methyl ester (L-NAME). Materials and Methods: Twenty-eight, 160- 220 gram, three to four month old female Sprague-Dawley rats were used in this study. After conception was confirmed by vaginal smears on the first day of pregnancy, the animals were allocated into four groups according to the chemicals fed in their drinking water as control (nothing administered), L-NAME (50 mg/kg L-NAME), MgSO₄ (600 mg/kg MgSO₄), and MgSO₄ + L-NAME group (600 mg/kg MgSO₄ + 50 mg/kg L-NAME). The pregnant uterus strips were isolated on the 19th day and the contractile activity of uterus was examined by applying 0, 0.1, 0.2, 0.4, 0.8, and 2.5 mIU/ml oxytocin to each group and responses are recorded accordingly. Results: There were no statistically significant differences regarding fetal parameters and peak amplitudes of the oxytocin stimulated pregnant rat myometrial strips among groups. In L-NAME group at 0 and 0.1 mIU/ml oxytocin, the contraction frequency in a ten-min period was statistically lower than the control group (Z= -2.850, p = 0.004; Z = -2.902, p = 0.004, respectively). In MgSO₄ group only at 0 mIU/ml oxytocin, the frequencies of the contractions in ten-min period were statistically lower than the control group (Z = -2.973, p = 0.003). In L-NAME + MgSO₄ group at 0, 0.1 and 0.2 mIU/ml oxytocin concentrations the frequencies of the contractions in ten-min period were statistically lower than the control group (Z = - 4.018, p = 0.000; Z= -3.237, p = 0.001; Z= -2.902, p = 0.004, respectively). In L-NAME + MgSO₄ given group at each oxytocin concentrations, the frequencies of the contractions in ten-min period were lower but not statistically different than the L-NAME group. Conclusion: MgSO₄ has no significant effect on the amplitude of spontaneous or oxytocin induced myometrial contractions, but decreased the frequency of spontaneous contractions. At each doses of oxytocin, MgSO₄ has no significant effect on the frequency of contraction in a pregnant rat model of preeclampsia induced by L-NAME.