Posttranslational protein modifications by the addition of amino acids are reactions which occur in intact sciatic and optic nerves of rats. The nerves differ, however, in that 2 h after crush injury these reactions are activated in sciatic but not in optic nerves. As sciatic nerves will eventually regenerate, whereas optic nerves will not, we have proposed that the activation of these reactions is correlated with the ability of a nerve to regenerate. The current experiments examined the posttranslational addition of amino acids to proteins at times greater than 2 h after nerve crush, during sciatic nerve regeneration and optic nerve degeneration. We also examined the optic nerve for morphologic correlates to changes in protein modification and partially characterized the proteins modified by [3H]Lys in the regenerating sciatic nerve using two-dimensional sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE). In a segment of sciatic nerve taken from a region just proximal to the site of crush, protein modification by covalent addition of [3H]Arg, [3H]Lys and [3H]Leu increased during both posttraumatic (2 h postcrush) and regenerative (6 days and 14 days postcrush) stages. Two-dimensional PAGE of [3H]Lys modified sciatic nerve proteins 6 days after crush injury showed labeling of proteins having molecular masses in the 18,000- to 20,000-, 30,000- to 40,000-, and 80,000- to 100,000-Da ranges, with neutral or basic isoelectric points (pI 7.1 to 8.0). In the retinal portion of the crushed optic nerve, incorporation of the same amino acids was unchanged or depressed to 21 days postcrush, except at 6 days postcrush when the incorporation of all three amino acids into proteins was increased threefold. These increases correlated with the appearance of terminal end bulbs in the portion of nerve analyzed. Histological examination of each nerve 2 h postcrush showed marked edema in the optic but not the sciatic nerve, a condition which may be related to the ability of sciatic and inability of optic nerves to activate protein modification reactions.