AbstractCarp respond to water hypoxia with an evaluation in the rate of gill ventilation. In order to characterize closer the adequate stimulus for the increase in respiratory drive specimens of carp,Cyprinus carpioL., were exposed simultaneously to moderate environmental hypoxia (PwO2˜ 75mmHg, 10kPa) and elevated water nitrite concentration (089 ± 0‐lmmol/l) for 24h. The differential effects of these treatments were utilized to distinguish between the responses to an immediate reduction in water and arterial Poa(P,O2), and to the slowly developing reduction of arterial oxygen content (CaO2) and functional oxygen saturation (SaO2). After onset of hypoxia gill ventilation quickly increased, leading to a reduction in PaCO2. Slowly rising blood methaemoglobin levels resulted in a gradual decline in CaO2and SaO2over 24h, whereas P002 remained steady for the entire exposure period. This pattern of lowered Paco2and PaO2, essentially constant for 24 h, together with the lack of any correlation with changes in CaO2, suggests PO2(Pa,O2and/or Pwo2) as the primary stimulus in the regulation of ventilation of c
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