The interaction between temperature and ethanol-induced fluidization was investigated in brain synaptic plasma membranes from C57BL/6 mice. Changes in fluidity were measured using the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene. Fluorescence polarization was tested in the presence and absence of ethanol at 25, 32 and 37 degrees C. An increase in temperature resulted in a significant increase in the baseline fluidity of the membranes and an increase in the magnitude of ethanol-induced fluidization of brain membranes. The combined effect of temperature on baseline fluidity and the magnitude of the response to ethanol resulted in a significant temperature-related increase in the relative response to ethanol (% change in polarization). The minimum concentration of ethanol required to cause a significant increase in the fluidity of the membranes was 170.7 mM at 25 degrees C and 85.3 mM at both 32 and 37 degrees C. The present results indicate that temperature-related changes in the effects of ethanol on membrane properties may underlie the effects of temperature on ethanol sensitivity in C57 mice.