We have shown differences in hormonal regulation of adenylate cyclase activity in fat cell ghosts prepared from rat, rabbit, fox and badger adipose tissue, under the influence of catecholamines, ACTH and insulin. a) In the rat, catecholamines induced a large stimulation (+315%) of adenylate cyclase. b) In the rabbit, ACTH was the most effective hormone. c) In the fox and the badger, only catecholamines could stimulate adenylate cyclase. d) In both rat and rabbit, insulin did not reduce spontaneous enzymatic activity. Moreover, the activation of adenylate cyclase by ACTH in the rabbit was not altered by insulin, while in the rat, this hormone slightly decreased epinephrine stimulation. 2. Hormonal regulation of adenylate cyclase correlated with the lipolytic response.