Endothelin (ET) is a potent and long - acting vasoconstrictor peptide consisiting of 21 amino acids, and recently isolated from a medium of cultured porcine endothelial cells. To study its tracer kinetics property, we injected 125I -labelled ET into rat veins. The results are shown that the half time of the distribution phase and that of the elimina-tion phase of 125I-ET in plasma are 5. 56 ?0. 04 min and 210. 13?38. 09 min, respectively. The125 - ET were widely distributed in various organs. The lung and kindney were the tissues with the highest grain density. Study of those in mouse blood and tissues showed similar results. KEY WORDS endothelin; tracer kinetics; dynamics; tissues; plasma