Effect of single selank administration on activity of carboxypeptidase H and of phenylmethylsulfonylfluoride-inhibited carboxypeptidase - enzymes detaching arginine and lysine from C-terminus of molecules-precursors of biologically active peptides was studied. The preparation has been shown to cause long, preserved for 24 h changes of activities of these carboxypeptidases. It is suggested that the change in activity of the studied enzymes can be one of mechanisms of regulation of level of neuropeptides at action of selank.