Because sialic acid is a potential biochemical marker of membrane development or alteration, we compared acute and chronic ethanol effects on sialic acid. Experiments were conducted with 50 adult male Wistar rats (approximately 400 gm), housed in groups of five. Rats drank ad libitum a vitamin-fortified diet (Nutrament) that was adulterated with ethanol; ethanol intake averaged for each rat 10-18 gm/kg/day. Controls were fed an equal total volume, made isocaloric with sucrose. Rats were sacrificed weekly for four weeks, and an acute challenge dose of ethanol (2 gm/kg, intraperitoneally) was given 45 minutes before sacrifice of both control and ethanol-consuming rats. Some controls were challenged only with saline. We replicated our earlier findings of regional differences in sialic acid and in cerebellar deoxyribose (measured as a necessary adjunct in the autoanalyzer modification of the Warren-Delmotte methods). In the saline-challenged controls, levels of both compounds were higher at four weeks than after one week. Similar increases occurred also in the chronic ethanol-consuming group, but not in the ethanol-challenged controls, which had significantly lower values. Results in saline-challenged controls suggest that the chronic treatment either 1) created a tolerance which protected cells from damage by the challenge dose of ethanol, or 2) killed neurons, thus promoting proliferation of glial cells.