Dormice (Glis glis) undergo spontaneous cyclic changes in food intake and body weight. These infradian cycles with a periodicity of about 2 mo are endogenously controlled, since they persist in conditions of constant temperature and photoperiod. To evaluate the role of insulin as an effector of hyperphagia and fattening in dormice, experiments were conducted to study pancreatic function and adipose tissue metabolism during several phases of the infradian cycle. During the weight loss phase, peripheral insulin resistance occurs in the absence of hyperinsulinism. This resistance is not corrected by weight loss. Weight loss phase animals showed poor glucose tolerance and an impaired in vitro glucose-stimulated insulin secretion; these were not attributable to reduced pancreatic insulin content. Although basal glucose transport and basal, as well as insulin-stimulated, glucose utilization in isolated adipocytes were depressed during the weight loss phase, insulin-stimulated transport was significant. The data offer no evidence that insulin has a direct causal role in the development of spontaneous obesity in this species.