The effect on labor of administering ritodrine after a premature rupture of the membranes (PROM) was studied in a double-blind trial in 30 patients. The patients were selected according to the following criteria: 28 to 36 weeks' gestation, only 1 fetus, cervix dilated 4 cm or less, and absence of pyrexia or other signs of uterine infection. Fourteen patients received ritodrine and 16 received a placebo. The 2 groups were statistically comparable. None of the patients receiving ritodrine delivered within 24 hours. The difference between the 2 groups was statistically significant with respect to the number of patients delivered within 24 hours (P < 0.05). However, after 24 hours, there was no difference between the groups as regards the length of pregnancy. The infections registered in the mothers and infants were few and easily controlled. The incidence of idiopathic respiratory distress syndrome (IRDS) was low in the study and allows no conclusions concerning the benefit of proloning the pregnancy for more than 24 hours after PROM.