Experiments were done on the tortoise, Testudo hermanni (anesthetized), or on its lung-vagus preparation in which all other internal organs had been removed. By recording the afferent impulse patterns in fine strands of the vagus, three kinds of CO2-sensitive receptors were demonstrated. Heymans-type chemoreceptors: Afferents originating from thoracic arterial chemoreceptors fired randomly at amplitudes below 100 microV. Firing frequency increased when the animal was ventilated with CO2-enriched, nitrogen or hypoxic mixtures, or was given NaCN. Intrapulmonary CO2 receptors (IPC). In an artificially ventilated tortoise and in lung-vagus preparations where the lung CO2-fraction (FCO2) was below 2.5%, single units with a spike amplitude above 200 microV fired regularly. When CO2-enriched air was inspired, impulse frequency decreased markedly, and when the lung was washed with air, it returned to the control frequency following the initial excitation. In lung-vagus preparations the impulse frequency did not change with lung inflation, was null for FCO2 greater than 2.5-3.5%, and was inversely proportional to lower FCO2's. Intrapulmonary CO2-sensitive mechanoreceptors: In anesthetized artificially ventilated tortoises, impulse bursts synchronized with inspiration decreased when the animal inspired 5 or 10% CO2 in air. In lung-vagus preparations, slowly adapting mechanoreceptor responses provoked by lung inflation were reduced when the lung was inflated with CO2-enriched air.