The mechanism of action of benzodiazepines and ethanol demonstrates that these agents can synergistically affect the central nervous system (CNS). The effects of both ethanol and diazepam are likely to depend on the time of the day when they were administered. Diazepam influence on ethanol-induced sleeping and hypothermic activity in mice as well as the influence of combined administration of these agents on spontaneous locomotor activity and coordination in mice (rota-rod) were investigated. Experiments were carried out in the light phase (10:00-12:00 h) and the dark phase (22:00-24:00 h). It was shown that ethanol-induced sleeping time was longer in the dark phase than the light phase, and that ethanol increased spontaneous locomotor activity both in the light and the dark. Ethanol-induced hypothermia was lower in the dark than in the light. Diazepam decreased locomotor activity more strongly in the dark phase than by day. It impaired the hypothermic action of ethanol in the light phase, but did not have such an effect in the dark phase. Diazepam prolonged ethanol-induced sleep in the light phase, enhanced its action on locomotor coordination and decreased the stimulating effect of ethanol on spontaneous locomotor activity in mice. The chronobiological effect of the interaction between diazepam and ethanol seems to be of practical importance (sleep and motor coordination).