The synthesis of the photoreactive neurohypophyseal hormone analog [3-(p-azidophenylalanine]arginine vasopressin [( Phe(p-N3)3]AVP is described. [Phe(p-N3)3]AVP exhibits a rat antidiuretic activity of 173 +/- 18 U/mg and a high binding affinity for the renal V2 vasopressin receptor in bovine kidney; an apparent dissociation constant KD of (1.3 +/- 0.2) X 10(-8) M was determined. In the toad bladder [Phe(p-N3)3]AVP was the most potent photoreactive vasopressin analog studied to date. It stimulated urea transport to the same maximal value as AVP with an ED50 of (3.1 +/- 0.7) X 10(-8) M. After irradiation with UV light, [Phe(p-N3)3]AVP bound irreversibly to toad bladder receptors and generated a persistent increase in bladder permeability to urea and to water. Analogs of 1-deamino-vasopressin with a photoreactive aryl azido group either in position 4 or 8 of the vasopressin sequence retain substantial rat antidiuretic activity. Compounds with a photoreactive group in position 8 showed a low activity in the isolated toad bladder in the dark; but on exposure to UV light, the activity of these analogs increased both with respect to urea and water transport, and the permeability response persisted during prolonged periods of washout. These studies provide evidence that analogs of vasopressin with an azido moiety in position 3 or 8 bind covalently to toad bladder receptors and produce an irreversible stimulation of transport processes.