The three-dimensional media structure of the umbilical arteries in their various functional states has up to now been object of numerous investigations, which are predominantly based on light-microscopic findings and have come to extremely differing results. For that reason the electron microscope method with an optically higher resolution was chosen for this thesis. In total, 20 umbilical cords were recovered, and in each case one of the two umbilical cord arteries was kept in a condition of non-contraction by means of perfusion fixation immediately after the omphalotomy. After the necessary preparation longitudinal, diagonal and cross-sections of both non-contracted and fully contracted umbilical arteries as well as of the folds of Hoboken, which occur in the arteries postnatally, were investigated by the scanning electron microscope. During this examination the following findings were made: The media in the wide umbilical cord artery consists of two layers. The outer layer accounts for some three quarters of the wall and is composed of lamellae of parallel muscle fibres which surround the vessel alternatively in gently rising and falling thread-like turns. The inner layer accounts for the remaining quarter and also consists of lamellae the thread-like turns of which, however, run at a much higher pitch. Exactly subendothelial, a very thin layer of irregularly arranged fibres with longitudinal main direction can be clearly identified. The folds of Hoboken are caused by local media contraction in the course of which the outer media restructures itself, thus being the main carrier of the fold.(ABSTRACT TRUNCATED AT 250 WORDS)