Ritodrine as the first-line drug in the treatment of established preterm labor has been supplanted in some centers by magnesium sulfate. To assess the relative efficacy and rates of side effects of these two agents, 120 patients were randomly assigned to receive one of these two drugs. Patients were included if they had intact membranes and met strict criteria for the definition of labor. In both groups excellent outcome was achieved, with 96.3% and 92.3% of patients receiving ritodrine and magnesium sulfate, respectively, obtaining a delay in delivery of greater than 48 hours. Side effects were comparable in both groups, although they tended to be more serious in the patients receiving ritodrine. In patients receiving both drugs together, the rate of side effects was 77% without a demonstrable benefit over a single agent. We conclude that ritodrine and magnesium sulfate are tocolytics of comparable efficacy and when used aggressively are highly successful in delaying delivery.