Uteroplacental vascular adaptation during pregnancy depends on retrograde endovascular migration of trophoblast in the uterine spiral arteries and their subsequent incorporation into the vessel wall. In the human, this migration process occurs in a step-wise fashion, starting with plugging of the distal ends of the arteries, followed by migration into the decidual and, after several weeks' delay, into the myometrial segments. The hypothesis is put forward that haemodynamical forces play an important regulatory role in this process. A mechanical signal transduction system should then be present within the trophoblastic cells to trigger their rheotactic behaviour. Since the condition of preeclampsia is characterized by restricted colonization of spiral arteries by trophoblast, the implications of this proposed regulatory system on the pathogenesis of the disease are considered.