The N-terminal dipeptide Tyr-d-Ala of a mu-selective agonist, dermorphin tetrapeptide (DT, H-Tyr-D-Ala-Phe-Gly-NH2) and delta-selective agonist deltorphin C (DEL-C, H-Tyr-D-Ala-Phe-Asp-Val-Val- Gly-NH2) was changed into an aminodiacyl moiety. The relevant synthetic step is a nucleophilic substitution of bromine from a chiral 2-bromopropanamide by the amino group of tyrosine, with overall retention of configuration. The resulting pseudo tetra- and heptapeptides I-VI were characterized for mu- and delta-opioid receptor binding properties using [3H]DAGO and [3H]DPDPE, respectively, and in a bioassay using guinea pig ileum (GPI) and mouse vas deferens (MVD). As a result of chemical alteration of N-terminal depeptide moiety, all synthesized analogs showed considerable reduction in opioid receptor affinity compared to mu- and delta-prototypes (500-fold on the mu-site, analog I, and 125-fold on the delta-site, analog IV). Interestingly, analogs I and IV showed moderate antagonist activity, respectively, on GPI and MVD, with pA2 values of 6.05 and 6.82. Analog IV did not exhibit the delta-antagonist potency and delta-selectivity of TIPP peptides.