The activity of acetylcholinesterase (AChE) in acetylcholine receptor (AChR)-enriched membrane vesicles isolated from electric organ of Torpedo californica exhibited a biphasic response to ethanol action. Below an ethanol concentration of 35 mM, AChE activity increased with increasing concentration of ethanol. At ethanol concentrations greater than 35 mM, the activity was found to decrease montonically. In contrast, ethanol (35-400 mM) increased the activity of soluble AChE. This biphasic behavior was consistent with the proposed important role of ethanol-membrane interaction. Microcalorimetric measurements revealed that the enthalpy change in acetylcholine (ACh) hydrolysis reaction was 586 J/mol in association with membrane-bound AChE in AChR-enriched membrane vesicles, as compared to -544 J/mol with the isolated soluble AChE. This discrepancy was attributed to the presence of membranes. Unlike its action on the enzyme activity, ethanol did not affect enthalpy change in ACh hydrolysis reaction catalyzed by either membrane-bound or soluble AChE. Comparison of results on activity and heat measurements suggested that the interaction of ethanol with membrane vesicles was nonspecific with no ethanol-induced membrane structural or conformational change.