Umbilical cord length varies considerably and the factors controlling cord length are unknown. Experiments in rat fetuses indicate that (1) restriction of fetal movements by oligohydramnios leads to short cord. The umbilical cords were significantly shorter in proportion to the duration or time of onset of the oligohydramnios. The mean cord length represented 65% of littermate control values when persistent oligohydramnios was induced on day 15, 71% for day 16 and 78% for day 17 (term day 21). (2) Suppression of fetal movements by curarization from day 18 on leads to short cords, irrespective of amniotic fluid volume. The paralyzed fetuses with polyhydramnios had a mean cord length representing 85% of sham operated controls, and those with oligohydramnios and paralysis had a similar mean cord length, 86% of controls. (3) Extra-uterine pregnancies with the fetuses free in the maternal abdominal cavity, yet attached to their umbilical cords, led to cords measuring 147% of littermate controls. (4) In contrast, when the extra-uterine fetuses were fixed to the uterine horn, close to the placental implantation site, with little or no stretch applied to the cord, the mean cord length was 90% od littermate controls. These results indicate that tensile forces on the cord secondary to fetal movements are important determinants of cord length.