Summary The response of normal adipose tissue to epinephrine (0.05 μg/ml incubation medium) is completely abolished by hypophysectomy of the donor rats. Treatment of hypophysectomized rats with crude, whole rat pituitary extract restored the lipolytic action of epinephrine to normal. Posterior pituitary extract and corticotropin replacement was without effect. Thyrotropin injections produced a partial, and growth hormone treatment a full restoration of the lipolytic action of epinephrine. Treatment of hypophysectomized rats with cortisol or cortico-sterone reestablished the sensitivity of their adipose tissue to epinephrine, but only when relatively large doses of these hormones were given. Physiological doses of triiodothyronine (1 μg/rat/day), however, restored the normal response to epinephrine. It is concluded that several endocrine factors can influence the lipolytic action of epinephrine but that their physiological importance cannot yet be quantitated. The technical assistance of Miss M. C. Hagney is gratefully acknowledged.