Rat brain (Na+ + K+)-adenosine triphosphatase is inhibited by ethanol (EtOH) in vitro, the inhibition being greater in the presence of norepinephrine (NE). Enzyme preparations from EtOH-tolerant rats show less inhibition by EtOH in vitro and less sensitization by NE. To investigate the mechanism of these changes, the enzyme activity of brain microsomes from tolerant and sucrose-control rats was measured at temperatures from 10-40 degrees C. Preparations from nonwithdrawn and 24-hr withdrawn rats were studied in the absence of in vitro additions, and in the presence of 1 microM NE, 50 mM EtOH or 440 mM EtOH separately, and of 1 microM NE + 50 mM EtOH. From Arrhenius plots of the results, the transition temperature (Td) was calculated by a method of successive approximations, and the activation energies were calculated from the segments above and below Td. Chronic EtOH treatment significantly decreased Td, but increased activation energies below Td. These findings suggest different effects on membrane matrix lipids than on boundary lipids adjacent to the enzyme. However, EtOH-tolerant preparations showed less effect of EtOH in vitro than did control preparations, on both Td and activation energies. Preparations from EtOH-tolerant and withdrawn rats behaved almost identically, indicating that the changes accompany tolerance and are not withdrawal effects. NE + 50 mM EtOH produced the same effects as 440 mM EtOH alone, in all preparations. EtOH tolerance reduced the sensitizing effect of NE. EtOH is interpreted as affecting both the boundary lipids and the apoenzyme itself.