Siberian hamsters (Phodopus sungorus) exhibit a naturally occurring, reversible seasonal obesity with body fat peaking in long "summerlike" days (LDs) and reaching a nadir in short "winterlike" days (SDs). These SDinduced decreases in adiposity are mediated largely via sympathetic nervous system (SNS) innervation of white adipose tissue (WAT), as indicated by increased WAT norepinephrine (NE) turnover. We examined whether SDs also increase sensitivity to NE-stimulated lipolysis. This was accomplished by measuring NE-and  3-adrenoceptor ( 3-AR) agonist (BRL-37344)-induced lipolysis (glycerol release) as well as NE-induced cAMP accumulation by inguinal, epididymal, and retroperitoneal WAT (IWAT, EWAT, and RWAT) in isolated adipocytes of LD-and SD-housed hamsters. SDs increased potency/efficacy of NE-triggered lipolysis in a temporally and fat pad-specific manner. Thus when WAT pad mass decreased most rapidly (5 wk of SDs), potency (sensitivity/EC 50) and efficacy (maximal response asymptote) of NE-stimulated lipolysis were increased for all WAT pads and also at 10 wk for IWAT compared with their LD counterparts. SD enhancement of lipolysis was similar for NE and BRL-37344 in IWAT adipocytes. These results, coupled with our previous demonstration that SDs upregulate WAT  3-AR mRNA expression, suggest that increased  3-ARs mediated the SD-induced increased NE sensitivity. NE-stimulated adipocyte accumulation of cAMP was greater after 5 wk of SDs for IWAT and EWAT and after 10 wk of SDs for IWAT compared with LDs, with no photoperiod effect for RWAT. Therefore, the SD-induced increase in SNS drive to WAT and increased sensitivity to this drive may work together to increase lipolysis in SDs.lipolysis; body fat; BRL-37344; body weight; sympathetic ENERGY IS PRIMARILY STORED as triacylglycerols in mammals. The vast majority of lipids are located in white adipose tissue (WAT), with considerably lesser amounts stored in brown adipose tissue (BAT) and other organs. WAT functions as a depot for energy storage, whereas BAT can dissipate chemical energy as heat through uncoupling of oxidative phosphorylation from electron transport in its mitochondria (for review see Ref. 8). During positive energy balance, food intake is greater than energy expenditure and lipid is deposited in WAT, whereas during negative energy balance, lipid is mobilized from WAT and transferred to BAT, muscle, and other tissues for oxidation. Chronic positive energy balance generates obesity, a condition that is not readily reversed in most mammals including humans. Obesity typically is associated with serious health risks that include the development of the metabolic syndrome, type II diabetes, and some cancers (35). There are, however, several species of mammals that exhibit seasonal obesity, with increasing adiposity at certain times of the year and decreasing adiposity at other times. Interestingly, although some of these obesities show characteristics similar to those associated with the negative consequences of obesity in humans, we are unaware...