We have compared the proteins from superior cervical sympathetic ganglia (SCSG) in young and old male Fischer rats. We have searched for the kinds of changes in protein size or charge which are predicted by such cellular aging theories as the error catastrophe theory, the somatic mutation theory, the cross-linkage theory, translational aging, the deamidation theory, and the autoimmune theory. SCSG from young (2 months to 1 year) and old (21–24 months) rats were labeled in vitro for 1 h with 14C-leucine prior to protein extraction for analysis. Protein analysis was on high-resolution, two-dimensional polyacrylamide gels using O’Farrell’s technique, with some modifications. Proteins were separated according to isoelectric point in one dimension and molecular weight in the second dimension. The changes predicted by the above aging theories were detected neither in the staining pattern, showing protein amounts, nor in the labeling pattern on autoradiographs of dried gels, showing newly synthesized proteins. The results suggest the possibility that, at least for rat, some other mechanisms for aging are pr