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An important goal of modern medicinal chemistry is to study the mechanisms of pain and analgesia in order to design effective analgesic drugs. Opioid analgesics are the gold standard for the treatment of severe pain; however, the use of opiates is associated with the development of side effects which are, in particular, related to the activation of the anti-opioid system. Mammalians synthesize a number of endogenous peptides, such as orphanin FGGFTGARKSARKLANQ, neuropeptide FF (FLFQPQRF-NH2), tripeptide melanostatin (MIF) PLG-NH2, as well as related compounds. These anti-opioid peptides are to one extent or another involved in homeostatic control of transmission of pain impulses. The present review includes the data published to date in domestic and foreign literature on the involvement of these peptides in such undesirable phenomena as inhibition of opioid analgesia, development of opioid tolerance and dependence, and hyperalgesia. Cell-cell and molecular ligand-receptor and receptor-receptor interactions of the opioid and anti-opioid systems are considered. These data can be useful for the design of new pharmaceuticals for pain relief. The generalization and study of these mechanisms are reflected in various approaches to treatment of pain syndromes and require analysis and further investigation. [ABSTRACT FROM AUTHOR]