The acute effects of ethanol on the nervous system are thought to be associated with disturbance of neural membrane function. In the present study the effects of ethanol, its immediate metabolite, acetyldehyde, and tertiary butanol which is not further metabolized to an aldehyde, on selected membrane-bound enzymes were examined in vitro in rat brain. The enzymes included acetylcholinesterase, succinate dehydrogenase, Na+K+-ATPase and cytochrome c oxidase. At concentrations ranging from 0.07 - 2% w/v (15 - 435 mM) ethanol did not produce significant inhibition of any of the enzymes tested. On the other hand acetaldehyde at concentrations ranging from 0.01 - 0.5% w/v (2 - 114 mM) showed marked inhibition of all the abovementioned enzymes except acetylcholinesterase. The responses of the various enzymes to tertiary butanol were intermediate between those obtained with ethanol and acetaldehyde. Further studies are in progress to evaluate the significance of these findings to the understanding of alcohol intoxication, tolerance and dependence in man.