Multidisciplinary research has recently identified an intrinsic appetite-regulating network (ARN) in the hypothalamus. The idea that viruses could help to chart this complex network has gained impetus owing to a combination of our improved understanding of virology and of genetic engineering. Recently, three groups have employed viral vectors as probes to: (1) trace the inflow of sensory information from the neocortex and limbic systems to the ARN; (2) trace the outflow of information from the ARN to the sympathetic nervous system to monitor adiposity and energy expenditure; and (3) decipher the mechanisms underlying leptin resistance, which is responsible for environmentally based obesity.