Eight analogues of Dyn A(1-11)-NH2 incorporating the enzymatically stable psi(CH2-NH) isosteric peptide bond replacement were synthesized and tested for binding affinity at the central opioid mu, delta, and kappa receptors in guinea pig brain (GPB) homogenates and for activity at the peripheral kappa (and mu) receptors in the guinea pig ileum (GPI). The peptidic analogues were synthesized by solid phase techniques using a Fmoc/tert-butyl strategy, and the psi(CH2-NH) bond, or reduced bond, was introduced via reductive alkylation of the N-terminal amino group of the growing peptide with a Fmoc-N(alpha)-protected amino aldehyde. The synthesis of Fmoc-N(alpha)-protected amino aldehydes also is described. Several other peptides have been previously synthesized incorporating this modification and showed for instance increased enzymatic stability and antagonist properties. Results obtained in the GPB show that modifications of the peptide bond in the address site (analogues 4-9) do not affect the binding at the kappa receptor and, with a few exceptions, at the mu and delta receptors. On the other hand, analogues 2 and 3, modified in the message segment of Dyn A(1-11)-NH2, show a decrease in binding affinity at all three receptors. In the GPI, the results are more varied as the influence of the peptide bond modification seems to be more important than in the GPB. Finally, selected analogues were tested with no indication for antagonist activity at the kappa peripheral receptor.