Three ketomethylene pseudodideptide analogues [(S)Lys psi(COCH2)(R and S)Phe (14 or 15 and 15 or 14) and (S)Lys psi(COCH2)(xi Trp (19)] of natural arphamenine A [(S)Arg psi(COCH2(R,S)Phe (1)] were easily prepared by a route involving two successive main reactions: a malonic ester alkylation with Z-protected lysine iodomethyl ketone and the introduction of a benzyl or (indol-3-yl)methyl moiety in position 2 of the resulting 4-ketodiester. The isomer of 1 with reversed sequence, (S)Phe psi(COCH2)(R,S)Arg (22) was synthesized by guanidylation and subsequent deprotection of Z-(S)Phe psi(COCH2)(R,S)Orn. The inhibitory effects of compounds 14, 15, 19, and 22, and the related ketomethylene dipeptides (S)Ala psi(COCH2)(R,S)Phe (3), (S)Phe psi(COCH2)(R,S)X [X = Ala (4), Orn (5)] and (S)Trp psi(COCH2)(R,S)Y [Y = Orn (6), Lys (7), Arg (8)] on aminopeptidase B (AP-B), and enkephalin-degrading enzymes [aminopeptidase N (APN) and neutral endopeptidase (NEP)] were compared with that of the model compound 1.