To observe the efficacy of magnesium sulfate in the treatment of preterm labor, 65 uncomplicated cases of preterm labor between 28 and 36 weeks of gestation were studied prospectively during Sep. 1988-May 1991. They were divided into two groups randomly. 30 cases were treated with magnesium sulfate and 35 cases with barbiturates or bed rest. The prevention of delivery for at least 48 hours after the initiation of therapy was achieved in 23 of the 30 cases (76.67%) of the magnesium sulfate group and in 3 of the 35 cases (8.57%) in the control group. Delay of more than 7 days was achieved in 17 of the 30 cases (56.67%) and in 2 of the 35 cases (5.71%). The postponement of delivery between the two groups. There was highly significantly difference (P less than 0.01). There was a significant correlation of cervical dilation at the onset of treatment to success of controlling preterm labor. In the magnesium sulfate group, the mean magnesium level to achieve tocolysis was 2.8 +/- 0.35 mmol/L (mean +/- s). The side effects to the mothers, fetus, and the neonates were mild and not prominent.(ABSTRACT TRUNCATED AT 250 WORDS)