The ether phospholipid PAF (1-alkyl-2-acetyl-sn-glycero-3-phosphocholine) was originally described as a potent bioactive lipid associated with inflammatory responses. More recently, we have suggested a functional role for PAF in fetal lung maturation and the initiation of parturition. Previously, PAF was detected in the amniotic fluid of women at term during labor. A significant proportion of this PAF was associated with a lamellar body-enriched fraction. In addition, the PAF concentration was elevated in fetal rabbit lung during the latter stages of gestation and in human fetal lung in organ culture. It was therefore concluded that the PAF present in amniotic fluid during labor was, in part, of fetal lung origin. In the reported study, amniotic fluid samples were obtained from a group of patients with uncomplicated pregnancies at term "not in labor" and "in labor" and a group with complicated pregnancies. We found an eightfold greater concentration of PAF in amniotic fluid obtained from the "term, in labor" samples compared with that of the "term, not in labor" samples (40 vs. 312 fmol/ml). A twentyfold elevation in PAF (838 fmol/ml) was evident in the amniotic fluid of patients incurring "preterm labor" with an average gestational age of 32 weeks compared with the "term, not in labor" group. PAF concentrations also were elevated to 756 fmol/ml amniotic fluid in a group of patients with an average gestational age of 31.5 weeks who had premature rupture of membranes. We have previously suggested that a second autacoid, PAF, in addition to eicosanoids, may function in the initiation of parturition at term in uncomplicated pregnancies. On the basis of our findings of a severalfold increase in the PAF concentration in amniotic fluid from complicated pregnancies, it is suggested that this potent activator of myometrial contraction, together with the eicosanoids, may be a contributing factor associated with premature labor.