1. Isolated brown fat cells from hamster respond to added catecholamines with a temporary increase in respiratory rate and an extended lipolysis. 2. From experiments with catecholamines and alpha and beta-blockers, the receptors of these cells are classified as beta according to classical definition. 3. Norepinephrine induces a rapid increase in adenosine 3':5'-monophosphate levels which parallels in time the stimulated respiration. Maximal adenosine 3':5'-monophosphate levels are reached within 1-3 min and are followed by a continuous decline. 4. Parallel to the catecholamine-induced respiration and lipolysis there is a pronounced drop in ATP levels. This energy depletion could be reversed by addition within 5 min after norepinephrine of the beta-blocker propranolol. 5. The nucleotide pattern in isolated hamster brown fat cells after norepinephrine addition was mimicked in experiments with isolated hamster brown fat mitochondria. From these experiments it is concluded that a high ratio of AMP and ADP over ATP decreases the respiratory rate when endogenous free fatty acids are oxidized.