The nonlinear interaction of homeostatic control loops appears to be at least as important as the function of the isolated loops in describing the response of man to stress. Although the medullary and carotid chemoreceptors have been investigated as controllers of respiration, a cogent description of how their effects interact is lacking. The objective of this proposed study is to provide a description of the interaction of these chemoreceptors in the form of a mathematical model of control of respiration. The constants of the controller equations will be determined by statistcal Parameter Estimation. Experiments will be performed on both awake and anesthetized dogs to separately stimulate the medullary and carotid chemoreceptors. The carotid sinuses will be auto-perfused by a servo-controlled, membrane oxygenator system to produce step changes of hypoxic prefusate and the medullary chemoreceptors will be stimulated by step changes of end-tidal PCO2. The direct experimental separation of the chemoreceptor stimuli coupled with rigorous statistical analysis is innovative and expresses the results of the study in an explicit mathematical statement.