Life requires that the brain accurately measure both internal (body core) and external (environmental) temperature. Environmental temperature detection allows animals to find suitable thermal climes and to perceive and learn to avoid painful stimuli. The mechanisms of environmental temperature detection are increasingly well understood, however the mechanisms by which the brain measures internal body temperature remain poorly defined. The vagus nerve is the dominant sensory system that monitors the state of the viscera, and vagal afferents are critical for commanding unconscious processes essential to life, such as keeping heart rate constant and controlling food digestion. But how the vagus nerve contributes to thermoregulation remains a mystery, and almost nothing is known about the specific molecules, cells, and pathways by which these afferents can trigger physiological and behavioural responses to temperature. I propose to identify the molecular and cellular mechanisms underlying thermodetection by vagal afferents innervating the gastrointestinal tract (GI) including the oesophagus, stomach and intestine. I will determine the molecular identity of vagal afferent cell types that measure core body temperature, determine the neurochemical signals that these thermosensitive vagal neurons use to communicate with the brain and test the hypothesis that these vagal cell types are essential for regulating temperature-dependent autonomic and behavioural functions in awake, behaving animals.