Abstract We measured plasma concentrations of endothelin-1 (ET-1), ET-3 and big ET-1 by sandwich-enzyme immunoassays in patients (Pt) with chronic hemodialysis (HD) (Pt-HD, n=23) and age-matched normal subjects (NS, n=17). In Pt-HD, plasma levels (before HD) of ET-1, ET-3 and big ET-1 were significantly higher than those in NS. Reverse-phase HPLC analysis indicated that plasma concentrations of ET-1, ET-3 and big ET-1 in both Pt-HD and NS can be precisely measured by these sandwich-enzyme immunoassays. In Pt-HD, although the plasma ET-3 or big ET-1 levels did not significantly correlate with blood pressure (BP), plasma ET-1 levels significantly (p<0.01) correlated with both the levels of systolic (r=0.63) and diastolic (r=0.54) BP. After 4-hour HD, the plasma level of ET-3, but not ET-1 or big ET-1, was significantly elevated and BP was significantly lowered. The present findings indicate that ET-1 and ET-3 play different roles in acute and chronic alterations of BP in Pt-HD.