Background: Several studies have shown that administration of magnesium sulfate prolongs the bleeding time. Objective: To investigate such effects in pregnant women in Shiraz, southern Iran. Methods:This study was conducted on 30 pregnant women aged between 18 and 32 yrs, with a gestational age of 20 to 37 weeks, who presented to Hafez and Zeynabiyeh Emergency Units affiliated to Shiraz University of Medical Sciences between January and August 1999, with premature labor pain or complaining of abdominal or back pain. The patients received tocolytic treatment with magnesium sulfate (MgSO4) in absence of any contraindication. The blood pressure and bleeding time of patients were measured upon admission to the labor room and before any intravenous infusion of MgSO4. Platelet count and serum magnesium level were also determined for each case. The aforementioned measurements and samplings were repeated following administration of 10 g MgSO4. Results: A significant difference was observed between the mean serum magnesium level and mean arterial blood pressure, before and after MgSO4 infusion. The mean bleeding time showed an increase of 27 seconds (15%) after infusion of Mg. Nevertheless, this value still remained in the normal range of 120–420 s. A direct relationship was found between the increase in serum magnesium level and the bleeding time. No change was observed in the mean platelet count. Conclusion:Magnesium therapy is associated with an increase in bleeding time in pregnant women, with no change in platelet count. This increase had no clinical significance and dose-independent.