The endogenous oxygen uptake and the uptake with added substrate of whole-brain and mid-brain homogenates were significantly lower in older lizards as compared to younger. Both whole brain and mid-brain showed responses to cold stress for different lengths of time but the decline in oxygen uptake was more pronounced in the younger age-group. Respiratory rate and response to cold stress were more marked in the mid-brain than in the whole brain. Basing on earlier reports on mammals and on the present findings, it is suggested that the decline in physiological adaptation in older animals may be a general feature in vertebrates.