Preeclampsia is a pregnancy-specific disorder characterized by hypertension and systemic endothelial dysfunction. Interleukin (IL)-1β is a possible mediator of maternal endothelial dysfunction in preeclampsia. Serum IL-1β as well as its natural inhibitor IL-1 receptor antagonist (IL-1Ra) were reported to be increased in women with preeclampsia. In the current study, we addressed the role of the placenta in controlling the circulatory levels of IL-1β and its natural inhibitor IL-1Ra in preeclampsia, and the possible effect of magnesium sulfate (MgSO4) on these levels. Using an ex vivo placental perfusion system, placentas from preeclamptic (n=9) and normotensive (n=6) pregnancies were perfused in presence or absence of MgSO4. Perfusate samples were collected from the maternal and the fetal circulations of the perfusion system, and IL-1β and IL-1Ra were examined by enzyme-linked immunoassay (ELISA). Preeclamptic placentas secreted higher levels of IL-1β (P<0.001), and a tendentious higher levels of IL-1Ra, mainly into the maternal circulation, as compared with normotensive placentas, although no differences in IL-1β:IL-1Ra ratio were detected. However, there was only tendentious increase in the secretion levels of IL-1β or IL-1Ra into the fetal circulation of preeclamptic placentas, when compared with normotensive placentas. Administration of MgSO4 to preeclamptic placentas resulted in an attenuation of the increased secretion of IL-1β into the maternal circulation (P<0.001), and in a tendentious reduction in IL-1Ra. However, IL-1β:IL-1Ra ratio in preeclamptic placentas was not affected by MgSO 4. Interestingly, exposure of normotensive placenta to MgSO 4 resulted only in increased levels of IL-1Ra in the maternal circulation, without affecting IL-1β levels or IL-1β:IL-1Ra ratio. These findings suggest that the placenta may contribute to the elevation in serum IL-1β and IL-1Ra in preeclampsia by increased secretion of these cytokines into the maternal circulation, and that MgSO4 is able to attenuate this increased secretion of IL-1β, and possibly IL-1Ra, in preeclampsia.