Institut de Physiologie, ERA 412 CNRS, 2 rue F. Magendie, 31400 Toulouse, France (Received 20 January 1978) It has never been clearly shown that endogenous corticotrophin (ACTH) affects the release of storage fat. Desbals, Desbals & Agid (1970) observed significant mobilization of lipid within a few hours of ablation of the adrenal glands in the rabbit; this response was prevented by hypophysectomy and could be restored by administration of ACTH. A possible extra-adrenal action of endogenous ACTH on adipose tissue was postulated from these results, but the experimental conditions were very different from those found in normal animals. This study was undertaken to investigate the physiological role of ACTH in the mobilization of fat in the rabbit. The rabbit was selected because of the extreme sensitivity of its adipose tissue to the lipolytic action of ACTH and its very slight responsiveness towards the lipolytic action of catecholamines in vivo (Desbals