Considerable evidence supports a central role for prostaglandins in the genesis of uterine activity and cervical dilatation at the time of human labour. The first step in prostaglandin synthesis is the liberation of arachidonic acid from glycerophospholipids by phospholipase A2. In this study, we examined the periparturitional gene expression of PLA2 in human placentae and fetal membranes, using a cDNA clone for non-pancreatic PLA2. PLA2 gene expression was found to be significantly increased (P less than 0.05) in placentae obtained after spontaneous-onset labour and normal vaginal delivery as compared with placentae from elective Caesarean section. In both of these groups, PLA2 expression in amnion and chorion was significantly less than that in placenta. The results of this study suggest that placental PLA2 gene expression increases in association with spontaneous-onset of labour in the human. This conclusion is consistent with the finding of increased levels of prostaglandins in gestational tissues, amniotic fluid and blood plasma at the time of labour.