Comparative study of hydrolysis of carnosine and a number of its natural derivatives by human serum and rat kidney carnosinase was carried out. The rate of carnosine hydrolysis was 3-4-fold higher then for anserine and ophidine. The rate of homocarnosine, N-acetylcarnosine and carcinine hydrolysis was negligible by either of the enzymes used. Our data show that methylation, decarboxylation or acetylation of carnosine increases resistance of the molecule toward enzymatic hydrolysis. Thus, metabolic modification of carnosine may increase its half-life in the tissues.