Lipolysis was studied in subcutaneous fat from obese and lean individuals. Glycerol was released at similar rates using pieces of fat weighing 40 or 100 mg. Isolated fat cells offered no advantage over segments of fat for the study of lipolysis. The lipolytic effects of epinephrine and norepinephrine were potentiated by phentolamine (an α-adrenergic blocking drug) and blocked by propranolol (a β-adrenergic blocking drug). Isoproterenol-stimulated lipolysis was also blocked by propranolol but was not potentiated by phentolamine. Thyrotrophin and a lipid-mobilizing factor from human pituitaries were lipolytic in vitro. Human growth hormone with or without dexamethasone had no effect on lipolysis.