Endogenous opioids have been studied extensively since their discovery, in the hope of findings a perfect analgesic, devoid of the secondary effects of alkaloid opioids. However, the design of selective opioid agonists and or antagonists has proved very difficult. First, structural studies of peptides in general are hampered by their intrinsic flexibility. Second, the relationship between constitution and the socalled “bioactive conformations” is far from obvious. Ideally, a direct structural study of the complex between a peptide and its receptor should answer both questions, but such a study is not possible, because opioids receptors are large membrane proteins, difficult to study by standard structural techniques. Thus, conformational studies of opioid peptides are still important for drug design and also for indirect receptor mapping. This review deals the pharmacological activity of : a) a new μ and δ agonist: The single amino acid replacement of 2,6-dimethyl-L-tyrosine in deltorphin B (H-Dmt-DAla- Phe-Glu-Val-Val-Gly-NH2) yielded high affinity for mu- and delta-binding sites. [Dmt1]Deltorphin B lacks activity at kappa-opioid binding sites. Bioactivity in vitro with guinea-pig ileum confirmed that [Dmt1]deltorphin B interacted with mu-opioid receptors by reducing electrically induced contractions in a naloxone-reversible manner and was 150-fold more potent than morphine and comparable to [D-Ala2,NMePhe4,Gly-ol5]enkephalin (DAGO). The inhibition of spontaneous contractions of rabbit jejunum provided evidence for delta-opioid receptor interaction. Analgesia (hot plate and tail flick tests) revealed that [Dmt1]deltorphin B was 180- to 200-fold more potent than morphine. Pretreatment with naloxone, naltrindole or H-Dmt-Tic-Ala-OH (a highly selective delta-opioid receptor antagonist) prevented [Dmt1]deltorphin B antinociception. Thus, [Dmt1]deltorphin B exhibited remarkably high dual affinity and bioactivity toward deltaand mu-opioid receptors. b) two new δ opioid peptide receptor antagonists (Dmt-Tic-OH (DTOH) and Dmt-Tic-Ala-OH (DTAOH): Dmt-Tic- OH (DTOH) and Dmt-Tic-Ala-OH (DTAOH), effective antagonists in vitro, represent a new potent opioid dipeptides for the delta-opioid receptor (Ki delta of 0.022 nM and a selectivity, Ki mu/Ki delta, of 150,000 for DTOH; Ki delta of 0.285 nM and a selectivity Ki mu/Ki delta, of 20,4 for DTAOH). In the present study we considered the pharmacological activity of these two new delta opioid peptide receptor antagonists in vivo. Therefore, we have evaluated their possible antagonistic activity against the antinociception induced by the highly selective delta opioid receptor agonist, [D-Ala2]deltorphin II (DEL). Furthermore, these two delta opioid peptide receptor antagonists were injected centrally or peripherally in order to assess their ability to act also after systemic administration. Concurrent i.c.v. injection of DTOH or DTAOH (0.5-1.0- 2.0 nM) with DEL (5 nmol) induced a significant reduction of DEL antinociception. By contrast, while DTOH (10-20-40 mg/kg) administered peripherally (i.p., s.c. or i.v.) was also able to reduce DEL antinociception, DTAOH failed. The present results indicate that DTOH is the first opioid dipeptide with delta antagonist activity after systemic administration and it could be important in the clinical and therapeutic applications. c) a new μ selective opioid dipeptide antagonists: the potent delta selective opioid antagonist dipeptides were designed on the basis of a simple conformational analysis. Following a similar procedure we found a mu selective dipeptide antagonist, 2,6-dimethyl-Tyr-D-Phe-NH2. Although its selectivity is not as high as those of the quoted delta selective dipeptides it has good in vitro activity and looks very promising for further development since the 2,6-dimethyl-Tyr-D-Phe message, like the delta selective 2,6-dimethyl-Tyr-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid counterpart, seems able to impart antagonism to longer peptides.