The syncytiotrophoblast contains aggregates of nuclei termed syncytial knots. Increased numbers of syncytial knots have been reported in placentae of pregnancies complicated by pre-eclampsia and fetal growth restriction (FGR). As oxidative stress has been implicated in the pathophysiology of these disorders, we hypothesised that the formation of syncytial knots may be induced by exposure to hypoxia, hyperoxia or reactive oxygen species (ROS). We assessed both the number and morphology of syncytial knots induced by culture in hypoxia, hyperoxia and with ROS. We also investigated whether the presence of syncytial knots in normal tissue was associated with a down-regulation of anti-apoptotic proteins Bcl-2, Mdm2, XIAP and survivin. Using our measurement system we describe an increased number of syncytial knots when tissue is cultured in hypoxia, hyperoxia or in the presence of ROS. The morphology of these syncytial knots was similar to those seen in vitro, although the nuclei from cultured placental explants were morphologically more homogenous, had fewer nuclear pores, and a higher heterochromatin:euchromatin ratio. Despite the apoptotic appearances of nuclei we did not detect a loss of anti-apoptotic proteins in the region of syncytial knots. We conclude that the increased number of syncytial knots in placentae from pregnancies complicated by pre-eclampsia and FGR can be replicated in vitro by ROS or hypoxia, supporting their involvement in the pathogenesis of these conditions. © 2006 IFPA and Elsevier Ltd.