Endocannabinoids have been implicated in the mechanisms of implantation, maintenance of pregnancy and parturition in women. Intrauterine prostaglandin production and actions are also critical in each of these mechanisms. Hence, we have evaluated the effects of cannabinoids on prostaglandin biosynthesis by human gestational membranes. Explants of term amnion and choriodecidua were established and treated with endogenous endocannabinoids 2-arachidonoyl glycerol (2AG) and anandamide (AEA), as well as the synthetic cannabinoid CP55,940, to determine their ability to modulate prostaglandin E2 (PGE₂) production. The explants were also treated with CP55,940 in the presence of either SR141716A (a potent and selective antagonist of the cannabinoid receptor CB1) or NS398 (a COX-2 inhibitor), to determine whether any observed stimulation of PGE₂ production was mediated through the CB1 receptor and/or COX-2 activity. All three cannabinoids caused a significant increase in PGE₂ production in amnion but not in choriodecidua. However, separated fetal (chorion) explants responded to cannabinoid treatment in a similar manner to amnion, whereas maternal (decidual) explants did not. The enhanced PGE2 production caused by CP55,940 was abrogated by co-treatment with either SR141716A or NS398, illustrating that the cannabinoid action on prostaglandin production in fetal membranes is mediated by CB1 agonism and COX-2. Data from western blotting show that cannabinoid treatment results in the up-regulation of COX-2 expression. This study demonstrates a potential role for endocannabinoids in the modulation of prostaglandin production in late human pregnancy, with potentially important implications for the timing and progression of term and preterm labour and membrane rupture.