One of the hallmarks of aging is the decreased capacity to adapt to changes in the environment. In a constant, controlled environment, it is often difficult to distin guish the functioning of the young organism from that of the old. However, when stresses are applied-temperature stresses, dietary stresses, infection, emotional stresses-the differences are more apparent. Frequently, the response ofthe old is delayed and may never reach the same mag'nitude as the young. These responses, in most cases, are mediated by the neuroendocrine system, which communicates via hormones, neurotransmitters,...