(S)-3-Phthalimido- or (S)-3-tritylaminolactams, prepared from L-lysine and L-ornithine in two or three steps, are easily methylated at the endocyclic nitrogen atom by iodomethane/silver(I) oxide in dimethylformamide. Acid hydrolysis of the N-methylated lactams thus obtained affords N6-methyl-L-lysine and N5-methyl-L-ornithine hydrochlorides in high yield.