The special topics of the workshop focused on leptin mechanism in ruminant and poultry. One of the essential roles of this hormone is to inform the organism about the level of fat reserves. The leptin gene is expressed in ruminants and poultry adipose tissues. Recent results on variations in plasma leptin and/or levels of leptin mRNA in adipose tissues show positive effects of body fatness and feeding level, and an inhibitory beta-adrenergic effect in cattle. In other respects, in vitro leptin production is stimulated by glucocorticoids and insulin, whose effects are inhibited by growth hormone. Progress in knowledge about leptin will allow to better understand and control the adaptations of energy metabolism and reproductive activity of ruminants to seasonal variations in day length and food supply, as well as variations in carcass fatness of growing animal.