Department of Medicine, University of California, San Diego, La Jolla, California 92093-0623 Chronic exposure to hypoxia results in a time-dependent increase in ventilation called ventilatory acclimatization to hypoxia. Increased O 2 sensitivity of arterial chemoreceptors contributes to ventilatory acclimatization to hypoxia, but other mechanisms have also been hypothesized. We designed this experiment to determine whether central nervous system processing of peripheral chemoreceptor input is affected by chronic hypoxic exposure. The carotid sinus nerve was stimulated supramaximally at different frequencies (0.5-20 Hz, 0.2-ms duration) during recording of phrenic nerve activity in two groups of anesthetized, ventilated, vagotomized rats. In the chronically hypoxic group (7 days at 80 Torr inspired P O 2 ), phrenic burst frequency (f R , bursts/min) was significantly higher than in the normoxic control group with carotid sinus nerve stimulation frequencies >5 Hz. In the chronically hypoxic group, peak amplitude of integrated phrenic nerve activity (  Phr, percent baseline) or change in  Phr was significantly greater at stimulation frequencies between 5 and 17 Hz, and minute phrenic activity (  Phr × f R ) was significantly greater at stimulation frequencies >5 Hz. These experiments show that chronic hypoxia facilitates the translation of arterial chemoreceptor afferent input to ventilatory efferent output through a mechanism in the central nervous system. hypoxic ventilatory response; acclimatization; central nervous system