Kunz and Miller )1974) described a phenomenon called pacing in which FCO2 oscillations forced in the lungs of unanesthetized unidirectionally ventilated chickens cause one ventilatory movement per ECO2 oscillation. In the present study SO2 (0.08--0.32 vol%) added to the ventilating gas stream caused a loss of pacing in 11 of 12 birds. In 7 of the 11 birds in which SO2 was effective, pacing returned 3.2--7.0 min after removal of SO2. Intravenous infusion of a similar quantity of SO2 (dissolved in saline) did not block pacing. In a second series of experiments chickens were again unidirectionally ventilated and single nerve fiber recording was used to investigate the effect of SO2 on individual intrapulmonary chemoreceptors (IPC). Of necessity, this work was done on birds which were anesthetized and paralyzed. Discharge was abolished in 10 of 12 IPC by doses of SO2 similar to those which blocked pacing. In 6 of the 10 fibers in which SO2 was effective, discharge resumed after a mean period of quiescence lasting 7 min. It is concluded that pacing is mediated by intrapulmonary rather than systemic chemoreceptors.