The study of the initial effects of a sudden and brief replacement of air by pure oxygen has been proposed as a tool (Dejours' test) to determine the tonic influence that arterial (peripheral) chemoreceptors were exerting upon ventilation under previous normoxic conditions. Therefore, the acute ventilatory response to transient hyperoxia should be used to assess the level of hypoxic chemosensory drive. In spontaneously ventilated pentobarbitone-anesthetized cats, we observed that the degree of ventilatory depression provoked by hyperoxia was correlated to the degree of previous hypoxia. Minimal tidal volumes (V-T) or transient apnea were reached between second to fourth cycles after switching from 5% to 100% O-2 breathing. Continuous recordings of chemosensory discharges from one carotid (sinus) nerve allowed correlation of the falls in frequency of chemosensory discharges to the degree of hyperoxia-induced ventilatory depression and provided an accurate measure of the prevailing chemosensory drive of ventilation exerted during hypoxic steady-state conditions.