Recently, we have demonstrated that substitution of 1,2,3,4 tetrahyidroisoquinoline-3- carboxylic acid (Tic) in place of Gly2 in dynorphin A-(1-13)-NH2 (DYN) analogue (A) decreased the affinity to the kappa, delta, and mu receptors, and kappa selectivity. The doubly substituted analogue [2,6apos;-dimethyl-L-tyrosine (Dmt1)-Tic2]DYN (B) exhibited high delta-affinity (Ki=0.39 nM) while mu- and kappa-affinities were only an order of magnitude less (4-5 nM). Bioactivity of [Tic2]DYN peptide (A) on guinea-pig ileum and rabbit jejunum revealed potent delta- and kappa-antagonism thus indicating that the conversion from a kappa-agonist to antagonist occurred with the inclusion of Tic into DYN analogues, similar to the appearance of antagonist properties with delta-opioid agonists containing a Tic2 residue. The present study was undertaken to compare the k- and δ-opioid antagonistic activity of two [Tic2] DYN peptides (A and B) with naloxone a well known non selective opioid receptor antagonist. This comparison was performed by using the model of opioid withdrawal in vitro. Following a 4 min in vitro exposure to U50-488 H (10-8 M), a selective k opioid receptor agonist, the guinea-pig isolated ileum exhibited a strong contracture after the addition of naloxone (10-5 M). Also, following a 4 min in vitro exposure to deltorphin II (10-8 M), a selective δ opioid receptor agonist, the rabbit jejunum exhibited a strong contracture after the addition of naloxone (10-5 M). Results are expressed as percent of Ach contractions. In our study, we showed that in guinea pig ileum the peptide A (k opioid receptor antagonist) was able to induce a strong contracture at a concentration of 10-9 M when injected 4 min after U50-488H (10-8 M). Also, in rabbit jejunum the peptide B (δ opioid receptor antagonist) was able to induce a strong contracture at a concentration of 10-10 M when injected 4 min after deltorphin II (10-8 M). The results of our experiments indicate that both peptide A (k receptor opiod antagonist) and peptide B (δ receptor opioid antagonist) showed an antagonistic activity higher than naloxone.