Hormone sensitive lipase is thought to be the rate-limiting enzyme of lipolysis; phosphatidate phosphohydrolase that of lipogenesis. Both these enzyme systems were studied in vitro with the addition of epinephrine. The results show that: (a) hormone sensitive lipase is four times more active in rat epididymal fat than in sheep tail fat and (b) sheep omental fat is twice as active as tail fat. (c) Phosphatidate phosphohydrolase is twice as active in rat fat as in tail fat, which is half as active as omental fat. (d) Epinephrine had a stimulatory effect on hormone sensitive lipase and an inhibitory effect on phosphatidate phosphohydrolase, the peaks of both overlapping at 4 x 10-1 μg/ml in the case of sheep. This would indicate that the two processes occur simultaneously and suggests a very sensitive hormonally-regulated system.