1. In the eel (Anguilla anguilla L.), the dorsal and ventral aortas and the subhepatic vein were catheterized to provide an external circulatory shunt. PO2 in the shunted blood was continuously recorded using a polarographic measuring device. 2. Spontaneous interruptions in the eel's ventilatory activity result in rapid changes in Pa,O2. The period of these Pa,O2 oscillations is sometimes less than 1 min. However, the response time of the measuring system is so great under the conditions of these experiments that only slow variations of Pa,O2, of a period greater than 5 min, are faithfully recorded. 3. In view of the impractibility of technical improvement in the recording system, a data processing method was developed to reconstitute the real physiological variations from the distorted experimental recordings. 4. Through this method of data correction, it is demonstrated that apnoeic or active ventilatory phases in the eel's breathing pattern are not triggered by constant low values of Pa,O2. Other physiological applications are suggested.