Here a new approach of the elucidation of paths of proteolytic biodegradation of physiologically active peptides, based on the use of a peptide with isotopic label at all amino acid residues and the enrichment of HPLC samples with unlabeled peptide fragments in UV-detectable concentration, has been proposed. The method has been applied for the investigation of degradation dynamics of the neuroactive heptapeptide MEHFPGP (Semax) in the presence of plasma membranes, and cultures of glial and neuronal cells obtained from the rat basal forebrain. The splitting away of ME and GP, and formation of pentapeptides are the predominant processes in the presence of all tested objects, whereas the difference in patterns of resulting peptide products for glial and neuronal cells has been detected. In conclusion, the approach applied allows analyzing physiologically active peptide concentrations in biological tissues and degradation pathways of peptides in the presence of targets of their action.