High circulating concentrations of leptin in obesity are associated with an apparent loss of its characteristic anorexic action within the hypothalamic region of the brain. Central insensitivity to leptin may therefore contribute to the aetiology of this disease, and an increased understanding of the underlying mechanisms will identify potential means of prevention and/or therapeutic targets. Seasonal animals such as sheep and Siberian hamsters (Phodopus sungorus) exhibit annual photoperiod-driven cycles of appetite and body weight. Increased food intake and weight gain in long days (summer) are associated with high circulating leptin, and decreased intake and weight loss in short days (winter) with low leptin. Critically, these cycles are associated with reversible changes in sensitivity to leptin. High sensitivity is seen in short days and relative insensitivity in long days, demonstrated both in sheep given leptin centrally via intracerebroventricular cannulas and in hamsters given leptin peripherally. In addition, primary hypothalamic appetite-regulating targets for leptin (i.e. neuropeptide Y, melanocortin and cocaine- and amphetamine-regulated transcript pathways) respond differently in these species to changes in circulating leptin and nutritional status induced by photoperiod as opposed to such changes induced by food restriction. Studies of seasonal animals will help to resolve causes of altered sensitivity to leptin and whether these changes reflect altered transport into the brain and/or altered signalling at the receptor or post-receptor level. Increased knowledge of the mechanism(s) and time-course for development and reversal of reduced central leptin sensitivity will provide new insights into the development and control of obesity.