In the present study, we tested the hypothesis that chronic intermittent hypoxia (CIH) produces changes in the autonomic and respiratory responses to acute peripheral chemoreflex activation. To attain this goal, 3-week-old rats were exposed to 10 days of CIH (6% O(2) for 40 s at 9 min intervals; 8 h day(-1)). They were then used to obtain a working heart-brainstem preparation and, using this unanaesthetized experimental preparation, the chemoreflex was activated with potassium cyanide (0.05%, injected via the perfusion system), and the thoracic sympathetic nerve activity (tSNA), heart rate and phrenic nerve discharge (PND) were recorded. Rats subjected to CIH (n = 12), when compared with control animals (n = 12), presented the following significant changes in response to chemoreflex activation: (a) an increase in tSNA (78 +/- 4 versus 48 +/- 3%); (b) a long-lasting increase in the frequency of the PND at 20 (0.52 +/- 0.03 versus 0.36 +/- 0.03 Hz) and 30 s (0.40 +/- 0.02 versus 0.31 +/- 0.02 Hz) after the stimulus; and (c) a greater bradycardic response (-218 +/- 20 versus -163 +/- 16 beats min(-1)). These results indicate that the autonomic and respiratory responses to chemoreflex activation in juvenile rats previously submitted to CIH are greatly increased.