The oxygenation of endogenous cannabinoids (eCBs), 2-arachidonoyl glycerol (2-AG) and arachidonoyl ethanolamide (AEA), by cyclooxygenase-2 (COX-2) produces novel types of prostanoids, prostaglandin glycerol ester (PG-Gs) and prostaglandin ethanolamides (PG-EAs). However, the physiological function of COX-2 oxidative metabolites of eCBs is still unclear. Here we demonstrate that PGE2-G, a COX-2 oxidative metabolite of 2-AG, induced a concentration-dependent increase in the frequency of miniature inhibitory postsynaptic currents (mIPSCs) in primary cultured hippocampal neurons, an effect opposite to that of 2-AG. This increase was not inhibited by SR 141716, a CB1 receptor antagonist, but attenuated by an IP3 or MAPK inhibitor. In addition, we also examined the effects of other prostanoids, derived from COX-2 oxygenation of eCBs, on mIPSCs. PGD2-G, PGF2á-G and PGD2-EA, but not PGE2-EA and PGF2á-EA, also increased the frequency of mIPSCs. The eCB-derived prostanoid-induced responses appeared to be different from those of corresponding arachidonic acid-derived prostanoids, implying that these effects are not mediated via known prostanoid receptors. We further discovered that the inhibition of COX-2 activity reduced inhibitory synaptic activity and augmented depolarization-induced suppression of inhibition (DSI), whereas the enhancement of COX-2 augmented the synaptic transmission and abolished DSI. Our results, which show that COX-2 oxidative metabolites of eCBs exert opposite effects to their parent molecules on inhibitory synaptic transmission, suggest that alterations in COX-2 activity will have significant impact on endocannabinoid signaling in hippocampal synaptic activity.