The isosteric methyleneoxy psi (CH2O) function was employed as a novel peptide-bond surrogate and incorporated into sequences of two neuropeptides, substance P (SP) and enkephalin. A pseudopeptide analogue [pGlu6,Phe8 psi(CH2O)Gly9]SP6-11 (7) of SP related C-terminal hexapeptide [pGlu6]SP6-11 and two pseudopeptide analogues of [Leu5]enkephalinamide, [Tyr1 psi (CH2O)Gly2, Leu5] enkephalinamide (11) and [Gly2 psi (CH2O)-Gly3, Leu5]enkephalinamide (17), were synthesized. The N alpha-protected pseudodipeptidic units were incorporated in the appropriate peptide sequences by using conventional coupling methods in solution. Compound 7 was a potent agonist (EC50 = 4.8 nM) of substance P as compared to the parent peptide [pGlu6]SP6-11 (EC50 = 1.2 nM), in stimulating contraction of the isolated guinea pig ileum (GPI). Analogue 7 was more potent on the neuronal (NK-3) than on the muscular (NK-1) tachykinin receptors in the GPI as shown by the ratio of activities, EC50 (NK-1)/EC50 (NK-3) = 3.16, thus displaying an improved selectivity for the NK-3 tachykinin receptor subtype as compared to that of [pGlu6]SP6-11, EC50 (NK-1)/EC50 (NK-3) = 0.44. In the rat vas deferens (RVD) assay, a typical NK-2 system, the pseudopeptide analogue 7 was (EC50 = 2 microM) 10-fold more potent than the parent peptide and 20-fold less potent than eledoisin, an NK-2 selective tachykinin. The pseudopeptide enkephalin analogue 17 had low biological activity when tested in the electrically induced GPI (EC50 = 2.3 microM) and was inactive in the mouse vas deferens (MVD) assay. In the rat brain membrane (RBM) binding assay analogue 17 had low affinity (in the micromolar range) for both the mu and delta binding sites. In contrast, analogue 11 was a potent enkephalin agonist (EC50 = 30 nM), being equipotent to [D-Ala2, Leu5]enkephalinamide (DALE) in the GPI assay. In the MVD, analogue 11 showed a substantially reduced activity (EC50 = 92 nM), being about 10-fold less potent than DALE. In the RBM binding assay analogue 11 showed high affinity (in the nanomolar range) for both mu and delta binding sites with increased selectivity for the delta sites as shown by the ratio of the apparent affinities for both receptors, Ki (delta)/Ki (mu) = 2.1. The contribution of the modified peptide bonds in the mode of interaction of SP and enkephalin at their corresponding receptors is discussed.