A rate of luminol-dependent chemoluminescence was studied in placental tissues and membranes of premature fetuses (32-36 weeks). Stimulation of free radical oxidation in peripheral parts of placenta and its decrease in ruptures of fetal membranes appear to be among the pathogenic factors responsible for preterm labour. These impairments were accompanied by dissimilar alterations in content of lipid peroxidation products -- diene conjugates and malonic dialdehyde. Possible importance of free radical-dependent protein modification in development of the pregnancy pathology studied is discussed.