The effect of phase-shift in photoperiod on the mouse's locomotor activity, water and food consumption was studied. The mouse sensitivity to the toxicity of ethanol, as measured by duration of narcosis time, was assessed. Determination of cerebral activity of alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) was made in specific mouse and rat brain regions. Animals were exposed for short-term of a 12 hr light and 12 hr darkness schedule or to continuous darkness. Motility counts showed inconsistent profile followed by a reduced frequency of occurrences in mice housed in complete darkness compared to controls. Mice slept longer from controls under ethanol narcosis when housed for 42 days in darkness. These mice showed a reduction of cortex ADH and ALDH occurring 24 hr post narcosis. Rats housed under continuous darkness showed a transient induction of ADH in some of the brain regions studied from 27 to 84 day period compared to inhibition of ALDH and ADH in certain mouse brain regions after 37 days of similar environmental condition. The results indicate species-dependent darkness induced alteration of cerebral activities of ADH and ALDH. The increased sensitivity towards the toxicity of ET by housing under continuous darkness is suggested, and the implication of such rhythm changes is discussed.