Experiments on model peptides show that the rate of deamidation of asparaginyl residues depends strongly on the nature of neighboring residues. The natural distribution of glutaminyl and asparaginyl residues is ordered with respect to the biological lifetime of the peptides and the functional groups of the residues neighboring to glutaminyl and asparaginyl residues. The rates of deamidation of such amide peptides under physiological conditions could serve as useful timers of development and aging.