PIP: Focus is on the concept of the quantitative thermostat or setpoint as it evolved experimentally, not theoretically, from measurements of central temperature at the tympanic membrane and from measurements of physiological responses by human gradient layer calorimetry and continuous analysis of oxygen consumption. Heat-flow responses as they relate to stimuli by which they are evoked -- peripheral or central temperatures at thermoreceptive sites -- and discussed, and individual systems of reflex action are described. These systems are then correlated with what is known of their tangible components -- sensory receptors, effector organs, nervous centers, and afferent or efferent nervous pathways -- classically derived from anatomical studies, lesion experiments, recording of action currents, or electrical or thermal stimulation of pertinent structures. Such as correlation is a crucial test, and when the results are consistent, they provide independent, mutual confirmation via the 2 approaches. Doubts are then removed that lesions or interferences, which are unavoidable with most neurophysiological methods, may have produced artifacts. Conversely, calorimetry allows one to observe and analyze thermoregulation in process in a human subject without disturbing normal structures or functions other than by the stresses to which the body is made to respond. Prior to a description of experimental findings, the essential methods are reviewed. With the methods of gradient layer calorimetry, tympanic (eardrum) thermonetry, and indirect calorimetry, thermoregulatory responses -- by chemical overproduction of metabolic heat -- were observed in transient states as a result of a stimulus of cold impinging on peripheral neurons or warm-inhibition removed from central structures. With tympanic thermometry permitting for the 1st time om humans the continuous recording of central thermal stimulation, and with gradient calorimetry permitting the continuous recording (with insignificant inertial distortion) of the sweating responses in precisely controlled environments of a large variety and under conditions where complete evaporation of sweat is ensured by strong but smooth air convection, a new study of physical thermoregulation was initiated in 1958. The experimental results did not support the expection that sweating would follow the average temperature of the skin in the classical tradition, with skin warm-receptors as the hypothetical origin of exciting warm-impulses and cenral temperature as a participant prescribing the action of the "thermoregulator" in some as yet undetermined, additive, manner. Vasomotor mechanisms to human thermoregulation can be assessed in quantitative manner only by whole-body calorimetry. Mechanisms of thermal homeostasis also considered include hormonal regulation, transmitter substances, conscious sensation of warmth and coordinations of behavioral and autonomic mechanisms, setpoint and its variations, and fever.