Collared lemmings are able to double their body weight and body fat content in response to a change in photoperiod. This occurs without a change in food intake. Photo-induced changes in body weight will be used to examine the temporal relationships among changes in energy expenditure (resting and activity) and fuel partitioning. In this revised application, it is proposed that both leptin and corticosterone may play important roles in the seasonal changes in body weight and body composition. It is hypothesized that during weight gain: 1) A decrease in resting energy expenditure will be associated with the early phase of weight gain, and a decrease in energy expenditure associated with physical activity will be associated with weight maintenance after weight gain; 2) increased metabolic efficiency (decreased expression of uncoupling protein mRNA) will cause the decrease in resting energy expenditure during the early phase of weight gain; 3) the photoperiod-induced decrease in corticosterone concentration will decrease ob mRNA expression, which in turn will decrease uncoupling protein mRNA expression. The converse will also be studied during weight loss in this model system. The results will shed light on the role of energy expenditure and energy partitioning on the regulation of body weight and body composition. The basic scientific information gained from this model should be useful for understanding the role of metabolic perturbations in the etiology of human obesity.