Nonenzymatic deamidation rates for 52 glutaminyl and 52 asparaginyl pentapeptides in pH 7.4, 37.0 degrees C. 0.15 m Tris-HCl buffer have been determined by direct injection mass spectrometry. These and the previously reported 306 asparginyl rates have been combined in a self-consistent model for peptide deamidation. This model depends quantitatively upon peptide structure and involves succinimide, glutarimide and hydrolysis mechanisms. The experimental values and suitable interpolated values have been combined to provide deamidation rate values in pH 7.4, 37.0 degrees C. 0.15 m Tris-HCl buffer for the entire set of 648 single-amide permutations of ordinary amino acid residues in GlyXxxAsnYyyGly and GlyXxxGlnYyyGly. Thus, knowledge about sequence-dependent deamidation in peptides is extended to include very long deamidation half-times in the range of 2-50 years.