The role of the carotid body in the regulation of respiration and cardiovascular reflex is well established. under adverse conditions such as hemorrhage, shock, hypoxia and acidosis, the associated chemoreceptors are stimulated and contribute significantly to homeostasis. Yet neither the mechanism which causes a receptor to discharge nor the structure of the receptor itself is known. To elucidate these features we will use a series of intracellular microelectrodes capable of marking the recorded sites in the carotid body of the cat. Tissue PO2, tissue pH, sinus nerve discharge and VO2 will be routinely measured. 1) The VO2 of the CB will be calculated during stopped flow from O2 disappearance curves. Subsequent histological analysis, including electron microscopic examination and histochemistry will identify the elements of the receptor complex. This information is essential for an understanding of the mechanism of chemoreception. 2) The above stopped flow experiments are expected to provide evidence for or against the Mills-Jobsis hypothesis that there are two types of cytochrome a3 in the CB. 3) Threshold TPO2 and TpH will be determined by recording the TPO2 and TpH at which sinus nerve discharge begins. These studies will show if discharge is associated with structures having low-affinity cytochrome a3 and will provide further evidence concerning a "barrier" to O2 diffusion in the CB. 4) The sympathetic nerves to the CB will be stimulated to ascertain whether the increased discharge is related to a lower TPO2. 5) Mediators known or suspected to be of importance in chemoreceptor function will be infused. Interrelationships among the measured parameters will be sought. 6) In vitro studies of isolated CBs in series will further elucidate the mediators involved in chemoreception. These studies are expected to yield valuable information not only on chemoreceptor discharge, but also on carotid body metabolism which appears to have some unique features.