The preeclampsia-eclampsia syndrome is a severe complication of the third trimester of pregnancy and represents the first cause of maternal death. It is mainly characterized by: weight increase, proteinuria and hypertension and can evolve with convulsions and maternal death. The etiology still remains unknown although a series of events have been identified, starting with endothelial damage and local vasoconstriction leading to hypertension. These events occur at first locally in the placental district and become generalized. This paper reports experimental and clinical data in order to demonstrate: 1) the presence of a substance that could evoke experimentally the damage present in this syndrome, 2) a mechanism that delivers such a substance to its primary action site, the placenta, and 3) the possibility to inhibit either the substance or the delivery mechanism in order to prevent this disease. Serotonin appears to play an important role in the chain of events leading to preeclampsia. Certain histological aspects, present in pregnant women with this type of hypertension, have been observed in experimental animals after the administration of serotonin. Platelet derived serotonin could be sufficient, in the case of endothelial damage, to determine vasospasm. In a condition of hypercoagulability, such as pregnancy, this situation can trigger a chain of mechanisms ending with renal damage. Low dose aspirin seems a valid therapeutic approach reducing thromboxane concentrations and therefore preventing vasospasm. In this way the pathogenetic sequence culminating in the preeclampsia-eclampsia syndrome is interrupted. Ketanserin inhibits the hypertensive potential of serotonin by selectively acting on S2 serotonin receptors and appears to be an effective treatment in this type of pregnancy induced hypertension.