- The effects of a number of neurohypophyseal hormones and analogues on the contractility of rabbit urinary bladder smooth muscle in vitro were examined. The order of potency was [Arg8]vasopressin = [Lys8]vasotocin greater than [Lys8]vasopressin greater than [Arg8]vasotocin much greater than [deamino1,D-Arg8]vasopressin greater than oxytocin much greater than pressinoic acid. The maximum tension induced by [Arg8]vasopressin was 4.4 N . cm-2 and the pD2 was 8.7. The effects of [Arg8]vasopressin were competitively antagonized by [(beta-mercapto-beta,beta-cyclopentamethylenepropionic acid)1,(O-methyl)Tyr2,Arg8]vasopressin and by [deaminopenicillamine1,Val4,D-Arg8]vasopressin with pA2s of 8.4 and 6.6, respectively. It was concluded that rabbit urinary bladder smooth muscle contains receptors for the neurohypophyseal hormones which have recognition properties similar to V1-vasopressin receptors.