The influence of opiate receptor antagonists: naloxone, naltrexone and diprenorphine, and of an agonist: morphine on ethanol-induced sleep, hypothermia and motor coordination impairment was investigated. Naloxone and naltrexone evidently antagonized the sleep and hypothermia, and improved the motor coordination impaired by ethanol. Naltrexone was the most, and diprenorphine the least potent ethanol antagonist. Morphine potentiated only ethanol-induced sleep, and this effect was opiate dependent (reversible by naloxone). The present results may partially support the hypothesis about participation of opioid system in ethanol action. However, ethanol effects were affected only by high doses of opiate antagonists, exceeding by far the doses antagonizing the morphine effects. Therefore the participation of additional, unspecific mechanisms, in addition to an opiate-mediated one, cannot be excluded.