Increased intrauterine prostaglandin (PG) production is crucial for the initiation of parturition. To investigate the mechanisms controlling intrauterine PG synthesis, we examined the expression of the key PG biosynthetic isoenzymes, PG-H2 synthase (PTGS)-1 and -2, in the amnion, visceral yolk sac (VYS), placenta and myo-endometrium of pregnant guinea pigs. This animal model was chosen because the hormonal milieu of pregnancy and the role of PGs in the hormonal control of parturition are similar to those in the human. PTGS1 mRNA abundance, measured by real-time RT-PCR, increased in the amnion and the placenta during the last third of gestation. During labour, PTGS1 mRNA levels decreased precipitously in all four tissues. PTGS1 protein abundance, assessed by immunoblotting, increased to high levels in the amnion and the placenta by the end of pregnancy and remained high during labour. PTGS2 mRNA expression was higher in the placenta than in the other tissues, but did not change before and during labour. PTGS2 protein expression decreased in the placenta and remained low in the other tissues during labour. Immunohistochemistry showed pervasive PTGS1 protein expression in the amnion and strong expression in the parietal yolk sac membrane (PYS) covering the placenta. PTGS2 was expressed in the PYS and the endometrium. The PTGS inhibitor piroxicam, administered in doses that inhibited PTGS1 but not PTGS2, significantly prolonged gestation. These data suggest that PGs generated by intrauterine PTGS1 are involved in the timing of birth in guinea pigs. The induction of PTGS1 in the amnion and the PYS is a critical event leading to labour in guinea pigs and models analogous changes in the human gestational tissues before labour.