The human amnion has become a significant tissue for metabolism of chemical substances with biologic activities, such as steroid hormones and prostaglandins. In this paper our studies on the biosynthesis of prostaglandins in human amnion focused on the role of prostaglandins in initiating and maintaining labor. First of all, the source of prostaglandins released in amniotic fluid was investigated. The amnion was demonstrated to possess the highest activity of generating prostaglandins among amnion, chorion laeve, and decidua vera, obtained after normal vaginal delivery. Subsequently, the bioconversion of arachidonic acid into prostaglandins and related compounds using the homogenate of human amnion was carried out without the addition of cofactors in the incubation mixture to show the natural spectrum of various types of PGs formed. PGE2 was revealed to be a main product of the amnion; its conversion rate was 5% of incubated radioactivity. Other compounds catalyzed by cyclooxygenase were PGF2 alpha (0.2%), TXB2 (0.3%), and HHT (1-2%), whereas 15-HETE (2-3%) and a mixture of 9, 11, 12-HETE (1-2%) were formed by lipoxygenase in the amnion. And third, the activity to produce PGE2 in the amnion obtained at normal vaginal delivery was greater than that obtained at cesarean section prior to the onset of labor. Thus, these results suggested that the amnion played a significant role in prostaglandin biosynthesis in uterine tissues. To provide a better understanding of the metabolism of arachidonic acid in the amnion as regards initiation and maintenance of labor, we discuss prostaglandin biosynthesis in the amnion and the mechanism of arachidonic acid release.