During fetal circulatory compromise, velocity pulsations in the precordial veins increase and are commonly transmitted through the ductus venosus into the umbilical vein, indicating a serious prognosis. The nature of the pulsations and their transmission into the periphery, specifically the umbilical vein, is poorly understood. We present information on the mechanical properties of fetal veins as a basis for describing the pulse wave propagation. Five fetal sheep livers with connecting veins (gestational age 0.8-0.9) were studied in vitro. The transmural pressure, obtained with a fluid-filled catheter, was reduced stepwise from 10.3 to 0 mmHg, and the diameter determined by ultrasonography. Each data set was fitted to an exponential function to determine the stiffness parameter and the area at a standard pressure, which we proposed to be 5 mmHg for the fetal venous circulation. The stiffness parameter was 6.2+/-1.8 at the ductus venosus outlet, 3.4+/-1.3 at the ductus venosus inlet, and 4.0+/-1.0 in the umbilical vein. Correspondingly, values for compliance and pulse wave velocity for the three venous sections were established for a physiological pressure range. The estimated pulse wave velocity of 1-3m/s is comparable with values estimated for veins in adults. The mechanical properties of fetal veins are comparable with those described for veins later in life. The stiffness parameter represents the elastic properties at all pressure levels and conveniently permits inference of compliance and pulse wave velocity.