Placental and renal cortical blood flow were measured by thermocouple method in 70 cases of late pregnant rabbits, 17 late pregnant dogs and 2 late pregnant monkeys. Intrauterine resting tonus in 148 primigravid and 74 multigravid women tended to increase towards the onset of labor, with maternal mean blood pressure elevating in close relation to the intrauterine pressure. Especially in primigravid women there was a strong tendency and intrauterine resting tonus was higher value in primigravidas than that in multigravidas. The increasing of intrauterine pressure by uterine contraction caused an elevation of maternal blood pressure by about 10% in pregnant women, and caused a decrease of placental blood flow by about 30% respectively, but it did not show any definite pattern in renal cortical blood flow in experimental animals. On the other hand, the increasing of intrauterine resting tonus with intraamniotic saline injection in 47 cases of experimental animals brought about a decrease of placental blood flow by about 30%, at the same time caused a decrease of renal cortical blood flow by about 20% and an elevation of maternal blood pressure by about 10% respectively. These findings support the concept that increasing of uterine resting tonus could play a role in mediating late pregnancy induced hypertension.