This study reports the synthesis of 1‐desamino‐7‐lysine‐ (fluorescein)‐8‐arginine‐vasotocin (7‐lys(flu) dAVT), and describes its biological activity in the isolated urinary bladder of the toadBufo marinus. 7‐lys(flu)dAVT was fully active in increasing bladder permeability to water. A half‐maximal hydroosmotic response was obtained at a concentration of 3×10−8M. A unique feature of this analog was that its reponse was not readily reversed after reoval of the analog from the serosal bathing solution. The residual response to 7‐lys(flu) dAVT was abolished (reversibly) by reducing serosal bath pH from 7.4 to 6.0, suggesting that acidification inhibits the response to analog at a step after the interaction of the ligand with its receptor. Although 8‐arginine‐vasopressin (AVP) was about 20 times more potent than 7‐lys(flu)dAVT in increasing membrane permeability to water, the response to AVP was readily reversed. Preincubation of bladders with 7‐lys(flu)dAVT in the presence of AVP blocked the residual response to 7‐lys(flu) dAVT. These studies suggest that 7‐lys(flu)dAVT forms a stable and physiologically active complex with hydrosmotic toad bladder receptors, and it may, therefore, serve as a useful fluorescent marker for receptors in tissues from this and other species that use vasotocin