The results of the study indicate a significant reduction in the ACh content in all the four regions of brain in rats after exposure to 30 min noise stress (Table 1). The F-test ratio (d.f.: 5, 30) shown by ANOVA in each area of the brain was as follows: cerebral cortex = 2.638 (p < 0.05), corpus striatum = 20.432 (p < 0.001), hypothalamus = 33.985 (p < 0.001) and hippocampus = 19.316 (p < 0.001).