The efficacy of magnesium sulfate was analyzed in relation to ritodrine hydrochloride. Patients presenting in preterm labor between 20 and 35 weeks' gestation were prospectively randomized. Tocolysis was achieved for more than 72 hours in 35 of 40 cases (88%) where magnesium sulfate was administered and 31 of 39 cases (79%) in which ritodrine hydrochloride was infused. Delay of greater than or equal to 7 days was achieved in 75% and 72% of cases, respectively. The mean dosage to achieve tocolysis was 4.5 gm/hr, in the magnesium sulfate group and 210.0 micrograms/hr in ritodrine hydrochloride-treated patients. The mean magnesium level to achieve tocolysis was 6.60 mg/dl. Side effects in the two groups were similar in number but less alarming in the magnesium sulfate group. Magnesium sulfate was found to be easy to administer and clinically efficacious. Its tocolytic action was found to be dose dependent and drug concentrations are easily determined. On the basis of this work and data from other investigators, magnesium sulfate should be used as the first line of tocolytic therapy with ritodrine hydrochloride as its pharmacologic backup.