The present study was designed to examine whether both induction and adaptation of brain Fos expression during acute stress depend on the intensity and duration of stressors. For this purpose, different durations of two types of acute stress, mild (restraint) and severe (immobilization) stress, were employed. Stress-induced Fos expression was analyzed quantitatively by immunohistochemistry. Adaptation of Fos expression to the acute stressors was not apparent in the hypothalamic paraventricular nucleus (PVN) or locus coeruleus (LC) but was observed in the amygdala, hippocampus, and cerebral cortex. A higher level of Fos expression was seen in the PVN, LC, and amygdala, following severe stress than was seen following mild stress. In the hippocampus, the dentate gyrus showed reduced Fos expression in response to stressors, although both mild and severe acute stress increased Fos expression in other regions of the hippocampus. The cingulate cortex showed increased Fos expression during mild stress, whereas long-duration severe stress reduced Fos expression. In the somatosensory cortex, both stressors increased Fos expression. These results indicate that the PVN and LC are relatively resistant to adaptation to acute stress compared to other brain regions. In addition, the PVN, LC, and amygdala may play important roles in the perception of the severity of stress.