Warm-blooded vertebrates regenerate lost limbs and their parts in general much worse than fishes and amphibians. We previously hypothesized that this reduction in regenerative capability could be explained in part by the loss of some genes important for the regeneration in ancestors of warm-blooded vertebrates. One of such genes could be , which encodes secreted protein disulfide isomerase of the Agr family. is activated during limb and tail regeneration in the frog tadpoles and is absent in warm-blooded animals. The essential role of another agr family gene, , in limb regeneration was demonstrated previously in newts. However, agr, as well as the third member of family, , are present in all vertebrates. Therefore, it is important to verify if the activity of lost by warm-blooded vertebrates is also essential for regeneration in amphibians, which could be a further argument in favor of our hypothesis. Here, we show that in the tadpoles in which the expression of or was artificially suppressed, regeneration of amputated tail tips was also significantly reduced. Importantly, overexpression of any of these or treatment of tadpoles with any of their recombinant proteins resulted in the restoration of tail regeneration in the refractory period when these processes are severely inhibited in normal development. These findings demonstrate the critical roles of and in regeneration in frogs and present indirect evidence that the loss of in evolution could be one of the prerequisites for the reduction of regenerative ability in warm-blooded vertebrates.