Triosephosphate isomerase (TPI) from mammals undergoes two specific deamidations (Asn-15 and Asn-71) which destabilize the isologous dimer and lead to the degradation of the protein. In aging cells and tissues, the deamidated isoforms accumulate apparently due to age-related changes in protein turnover. Chicken TPI lacks one of these sites (i.e., Asn 71----Lys), but also exhibits unstable isoforms. These isoforms are the result of the specific oxidations which occur both in vitro and in vivo. Electrophoretic analyses of various tissues from chicken show that the most oxidized isoform, which is present in adult tissues, is only present in small quantities in tissues of the newborn chick. Moreover, embryonic tissues contain almost exclusively the fully reduced form of TPI. Thus, it appears that oxidation rather than deamidation constitutes the first step in the degradation of avian TPI. TPI may be the first example of a protein which has evolved two different types of modifications (deamidation and oxidation) which trigger its degradation. The accumulation of both deamidated and oxidized isoforms in different species may provide clues to the underlying basis for the accumulation of modified proteins in aging.