Thirty-seven patients in premature labor with intact membranes were treated as follows. Ritodrine was given as a primary tocolytic agent and magnesium sulfate was added adjunctively when the uterine contractions could not be controlled even when the administration of ritodrine exceeded 250mmg/min (14 of 37 cases) (combination therapy group). Prolongation of pregnancy for more than 48 hours and delivery beyond 37 weeks were achieved in 89% and 68% respectively. These results were more satisfactory than those obtained with the isoxsuprine treated group (122 cases). The incidence of discomfort due to nasal obstruction was low in the ritodrine group, but side effects such as palpitation and hot flush occurred more commonly in the combination group. No life-threatening side effects were observed throughout this study. Twelve out of 37 cases (32%) were delivered before 37 weeks. Neonatal morbidity was more frequent in this group delivered before 33 weeks of gestation. These included respiratory distress, hypotension, hypoglycemia, and hypocalcemia. Moreover, two babies born from mothers treated with ritodrine and magnesium had ileus-like symptoms. These data suggest that this combination therapy is effective as far as the tocolytic purpose is concerned. However, special attention must be paid to adverse maternal and neonatal effects as well.