It is well known that ACTH/MSH-like peptides (melanocortins) apart from their classical endocrine effects exert pleiotropic non-endocrine actions. Heptapeptide Semax (MEHFPGP) is the analogue of ACTH(4-10) that has prolonged neurotropic activity in comparison to native peptide. Our previous data and clinic investigations have shown Semax analgesic activity and its influence on animal behaviour. The melanocortin and opioid systems have opposite activities, and have therefore been considered as functional antagonists. It has been reported that high concentrations of Semax antagonize MC4 and MC5 receptors in vivo and in vitro. The aim of the present work was to study the Semax influence on analgetic and anxiolytic effects of opioids. The present work was carrying out in white rats. Pain sensitivity was measured by using Randall-Selitto paw-withdrawal test. Anxiety-like behaviour was evaluated by the elevated plus maze test (EPM). Semax was administered intraperitonealy at dose 0,5 mg/kg. It was shown that Semax reduced pain threshold in paw-withdrawal test. Intraperitoneal pre-treatment with opioid receptors antagonist naloxone (1mg/kg, 15 min before peptide injection) failed to influence Semax analgesic effect, but naloxone (5 mg/kg) reduced Semax analgesia in paw-withdrawal test. Semax administration 15 min prior to opioid receptors agonist morphine (5 mg/kg) decrease morphine-induced analgesia. Semax decreased rat anxiety level in EPM. Also we have shown that Semax pre-treatment attenuates anxiolytic effects of morphine. These data suggest the melanocortin and opioid systems interaction. We can suppose that Semax effects on nociception and anxiety depends on the level of opioid system activity. The work was supported by the Program of Basic Research "Molecular and Cell Biology", Program Scientific School ([numero] 5638.2006.4) and Russian Fond of Basic Reseach ([numero] 07-04-00733).