Arginine-vasotocin (AVT) is a hormone common to most vertebrates except mammals where it is replaced by vasopressin. A study of the effect of both nonapeptides of the vasopressin family [AVT and arginine-vasopressin (AVP)] showed that in the rat kidney they are involved in the regulation of renal sodium and solute-free water reabsorption. These effects are mediated by different subtypes of Vreceptors activated by different blood serum concentrations of the hormone. An immunoenzyme assay (ELISA) was proposed to determine AVT concentrations under almost complete inhibition of AVP secretion. When both hormones were administered at equal doses, AVT broke down slower and acted longer than AVP. A high metabolic rate of the nonapeptide in the blood is supposed to be an evolutionary advantage as a tool to upgrade the water–salt balance regulation. The importance of comparing the effects of hormones lost during the evolution of vertebrates as a method to study the evolution of functions is discussed.