A sensitive and precise method for assaying the water permeability response evoked by neurohypophyseal hormones and their synthetic analogues on the isolated urinary bladder of the toad (Bufo marinus L.) is described. The method permits detection of 8-arginine-vasotocin at concentrations as low as 10-12M. This sensitivity, not achieved heretofore with this tissue, results largely from minimizing interference of inhibitory substances by means of an "in vitro circulation assembly." The precision of the method derives from a direct comparison between the cumulative dose-response curve of an agonist of unknown potency acting on one hemibladder and that of a reference compound acting on the contralateral hemibladder. Crystalline deamino-oxytocin is used as the reference standard in this assay. The intrinsic activity of 2-(O-methyltyrosine)-oxytocin, as defined by the maximal response, is 12% lower than that of deamino-oxytocin. All other hormonal peptides investigated have the same intrinsic activity as deamino-oxytocin, even 5-valine-oxytocin, in spite of its extremely low affinity. A comparison of the potencies of 8-arginine-vasotocin vs. 8-arginine-vasopressin, 8-ornithine-vasotocin vs. 8-ornithine-vasopressin, 8-alanine-oxytocin vs. 8-alanine-oxypressin, and deamino-8-alanine-oxytocin vs. deamino-8-alanine-oxypressin suggests that an isoleucine residue in position 3 imparts a higher specificity for binding of the hormonal peptide molecule to the bladder receptor than a phenylalanine residue in this locus.