We have suggested that a novel endothelin-1 with 31 amino acids [ET-I (1-31)] plays an important role in fetal circulation, owing to a strong contractile activity on the umbilical artery. To clarify the pathophysiological significance of ET-I (1-31) in the development of severe preeclampsia, its contractile activities on human umbilical arteries and uterine smooth muscle from patients with preeclampsia were studied. The contraction by ET-I (1-31) was stronger in uterine smooth muscle of the patients with severe preeclampsia than that of normal subjects. On the contrary, the constrict-ion of umbilical artery of the patients with eclampsia was significantly weaker than that of normal pregnant women. The stronger contraction of myometrium by ET-I (1-31) in patients with severe preeclampsia observed for the first time in the present study suggests that ET-I (1-31) might be involved in the development of preeclampsia.