The effect of ethanol on an enzyme system within an intact functional plasma membrane has been studied using neural cells grown in culture. Rat C6 glioma cells in mono-layer culture were treated acutely or chronically with 100 mM ethanol and the effect of this exposure on the activity of ecto-5'-nucleotidase was determined. Acute exposure led to an increase in enzyme activity with maximum stimulation occurring at concentrations of 100 - 400 mM ethanol. Chronic treatment of cells with 100 mM ethanol for 4 - 8 days also caused an increase in ecto-5'-nucleotidase activity. Both the acute and chronic ethanol-induced stimulation of enzyme activity was completely reversible by removing the ethanol; the acute effects reversed immediately, whereas the chronic effects required several hours. The addition of Concanavalin A demonstrated that the effects on enzyme activity of both chronic and acute exposure to ethanol were blocked by modification of the external cell surface. The effect of chronic exposure to 100 mM ethanol was further localized to an action on the plasma membrane by studies which showed chronic exposure to have no effect on the intracellular 5'-nucleotidase activity. Furthermore, the occurrence of pharmacological tolerance to acute ethanol was observed in this plasma membrane system following chronic treatment of C6 cells with 100 mM ethanol. These findings are consistent with the hypothesis that mammalian neural cells can adapt to the chronic presence of ethanol through changes in their plasma membrane.