The onset of labor in humans is associated with an increase in prostaglandin production. One of the key steps is the conversion of arachidonic acid to prostaglandin E2 by cyclo-oxygenase (Cox). Cox has been found to exist as two distinct genes, Cox-1 and Cox-2. We have used RT-PCR to study the relative abundance of mRNA from each Cox gene in amnion at term. Quantitation of PCR efficiency indicated an approximate 100 fold excess of Cox-2 messenger RNA over that for Cox-1. These data point to the importance of Cox-2 in the increased prostaglandin synthesis associated with labor. Further studies should therefore focus on the control mechanisms for Cox-2.