Lysine is an essential amino acid normally present in very low concentration in cereal seeds. In previous reports we have studied the metabolism of lysine in several distinct high-lysine maize mutants and observed drastic variations in the activity of saccharopine dehydrogenase (SDH), a key enzyme involved in lysine degradation. We have now analyzed the activity of SDH using non-denaturing polyacrylamide gel electrophoresis (PAGE) to identify possible isoenzymes that could explain the patterns of activity previously observed. The results indicated the presence of at least two SDH isoenzymes, one contributing to approximately 90% of the total enzyme activity and a minor form only present in the wild type lines and the opaque-1 mutant. The results suggest that the differences in total SDH activity among the genotypes tested are due to alterations in the predominant SDH isoenzymic form, which is likely to be the bifunctional polypeptide containing lysine 2-oxoglutarate reductase.