Large scale isolation and determination of amino acid sequences of endogenous peptides from various biological sources (bovine brain and red bone marrow, siberian ground squirrel brain) were carried out. A number of earlier unknown peptides were identified, many of them showing distinct activity in vivo and/or in vitro. Analysis of more than 170 isolated peptide structures resulted in the hypothesis suggesting functional proteins, in particular hemoglobin, to serve also as a source of the peptide "background" which has its own biological significance. Further directions of investigating the endogenous peptide material are mapped. Its potential for developing diagnostics for somatic diseases is demonstrated on patients with CNS disorders.