The ligand-dependent nuclear receptor PPAR gamma plays an important role in murine and human trophoblast differentiation. Oxidized lipids, which are implicated in the pathophysiology of placental dysfunction, have recently been identified as ligands for PPAR gamma. We therefore hypothesized that oxidized lipids activate PPAR gamma in human trophoblasts and influence placental function. To test our hypothesis, we examined the effect of 9S-hydroxy-10E,12Z-octadecadienoic acid (9-HODE), 13S-hydroxy-9Z,11E-octadecadienoic acid (13-HODE), and 15S-hydroxy-5Z,8Z,11Z,13E-eicosatetraenoic acid (15-HETE) on PPAR gamma activity in cultured term human trophoblasts. Our results demonstrate that these lipids stimulate PPAR gamma activity and that the AF-2 fragment, which harbors the ligand-binding domain of PPAR gamma, mediates this effect. Furthermore, we assessed the consequences of PPAR gamma activation by the oxidized lipids, and we found that these lipids stimulate human CG production, a measure of trophoblast differentiation. In contrast, the expression of syncytin, a marker for syncytium formation as well as the expression of the cell cycle modulators cyclin E and p27 are unchanged by the oxidized lipids. We concluded that 9-HODE, 13-HODE, and 15-HETE activate PPAR gamma in primary human trophoblasts. These PPAR gamma ligands may play a role in placental differentiation, yet they are unlikely to contribute to trophoblast dysfunction.