Objective: To investigate in vitro effects of ritodrine, magnesium sulfate and their combination on spontaneous contractions of myometrial strips obtained from pregnant rat uteri. Method: A total of 13 pregnant Sprague Dawley rats with weights between 180-200 g were used in this study. Three strips from each rat were kept in an organ bath containing 20 mL Krebs-Henseleit solution (pH: 7.4 and 37 ℃). 10^-8, 10^-6 and 10^-4 M concentration, of ritodrine, magnesium sulfate and the combination was applied over myoimetrial sctrips in Groups I (n:10), II (n:10) and III (n:8), respectively. Amplitude and frequency of spontaneous myometrial contractions, which were recorded at the beginning of each experiment, were considered as reference values. Amplitude and frequency changes in spontaneous myometrial contractions were calculated at approximately ten-minute intervals right after the application of drugs as the percentage of difference at the first reference response. Results: Magnesium sulfate application did not lead to any significant difference on the amplitude and frequency of contractions at any of its concentrations. 10^-6 and 10^-4 M concentrations of ritodrine caused a significant decrease in the amplitude of contractions. It was also found that ritodrine significantly decreased the frequency values at all concentrations. A significant decrease in amplitude was observed at 10^-8and 10^-6 M concentrations in the combination group. No significant decrease in frequency values was found at any concentration in the combination group. Conclusion: The tocolytic effect of ritodrine is superior to that of magnesium sulfate.