[1-beta-Mercapto-beta,beta-cyclopentamethylenepropionic acid),2-(O-methyl)tyrosine ]argine-vasopressin and [1-beta-mercapto-beta,beta-cyclopentamethylenepropionic acid)]argine-vasopressine, two highly potent antagonists of the vasopressor response to arginine-vasopressin.