Temperature of the gas stream and mucosa were measured in the upper and lower trachea and right and left main bronchi of several anesthetized, intubated and mechanically respired mongrel dogs. Airway temperatures were measured using an airway sensor probe instrumented with microthermistors. Each thermistor was integrated into an especially designed. Wheatstone bridge whose signal of millivolts was displayed on a calibrated polygraph recorder. Diving respiratory conditions were simulated by utilization of an appropriate ventilatory periodic flow through an endotracheal airway which by-passed the efficient gas conditioning nasal turbinates of the dog. Deep diving respiratory environmental conditions of gas temperature, density and thermal capacitance (ρCp) were simulated in a hyperbaric chamber. The temperatures recorded during in vivo periodic positive pressure ventilation were applied to a quasi-steady flow model based upon the morphological dimensions of the Weibel model. An empirical mathematical model of inspiratory sensible heat loss was verified and slightly modified to better reflect the overall dimensionless heat transfer relationship Nu = 0.302 (RePr)0.786 that existed in the major bronchial airways of the experimental subject. The design of the experimental instrumentation is explained in detail, as is the basic mathematical model. Significance of the experimental findings is discussed.