The paper concentrates on measurements of tissue pO2 and local flow, which are both assumed to determine the nervous signal of the carotid body. The measurements were performed with needle electrodes. Inside the specific tissue of the carotid bodies of cat and rabbit, which is assumed to be surrounded by an oxygen barrier, pO2 values between 7 and 25 torr were recorded. From experiments on the fetal carotid body it was concluded that these low values are essential for the chemoreceptive process. Two types of blood flow were observed in the carotid body: high flow running through arteriovenous shunt vessels or flow-through channels (total flow), and low flow running through the capillaries supplying the specific tissue (local flow). Local flow decreases under hypoxia, whereas total flow follows the changes in blood pressure in any case. These pO2-dependent regulatory mechanisms influence the extracellular calcium activity, which determines the calcium uptake of type I cells under hypoxia and, consequently, (by vesicle release) the chemoreceptor discharge. Other mechanisms are assumed to be involved during hypercapnia, since under these conditions local flow does not change.