The isosteric methyleneoxy 9 (CH20) function was employed as a novel peptide-bond surrogate and incorporated into sequences of two neuropeptides, substance P (SP) and enkephalin. A pseudopeptide analogue [pGlue, PhesO (CHzO) Gl~] SP,+ ll (7) of SP related C-terminal hexapeptide [pGlu6] SPbll and two pseudopeptide analogues of [Ledlenkephalinamide,[Tyr19 (CH20) Gly2, Leu6] enkephalinamide (11) and [Gly20 ( ...