Forty-three patients with preterm premature rupture of membranes were studied between 24 and 34 weeks' gestation. Intraamniotic infection was excluded by amniocentesis. There were significantly fewer fetal breathing movements for the first 2 weeks of membrane rupture, compared with controls, with a return to near normal by the third week. This reduction in fetal breathing may play a role in the production of pulmonary hypoplasia in some of these patients. Further studies are needed to investigate whether ruptured membranes are associated with a reduction in fetal breathing at earlier gestational ages.