Rabbit white adipose tissue, in vitro, may not respond to epinephrine by an increase in the rate of lipolysis although beta adrenergic receptors may be postulated from our experiments with isoproterenol. When basal rate of lipolysis is high, an antilipolytic effect may be shown for low concentrations of epinephrine (0.01 and 0.1 mug/ml). However, epinephrine and an alpha adrenergic blocker, phentolamine (5 mug/ml), acting together, abolish the antilipolytic effect and better still a strong lipolytic effect of epinephrine is unmasked in such a case. The best evidence of these results is that inhibitory alpha, adrenergic receptors are involved in Rabbit white fat cells. These facts partially explain the absence of lipolytic response with epinephrine previously described.