Quantitative mechanisms that regulate normal temperature and determine its clinical aberrations have been found with the methods of gradient calorimetry and tympanic thermometry. Homeostasis is achieved by warm-sensitive neurons of a "human thermostat" or "temperature-eye" in the anterior portion of the hypothalamus. These fire and excite sweating and vasodilatation for heat loss when their temperature exceeds a sharply determined set point near 37 C (98.6 F). Similar warm-sensitive neurons inhibit, through a synaptic station in the posterior portion of the hypothalamus, the metabolic response to cold which is excited by cold-receptors of the skin firing from thresholds at 33 to 35 C (91.4 to 95 F). Pyrogens depress or extinguish firing of central thermoreceptors. The set point is shifted. Fever develops. These findings provide a basis for the understanding of clinical temperature, and the tool for its reliable measurement—tympanic, not rectal thermometry.