1. Changes in ventilation volume (Vg) of rainbow trout caused by hypercapnia, hypoxia and anaemia were measured directly by collection of expired water. 2. Exposure to hypercapnic water (PCO2 range 0.5-2 kPa) increased Vg (by up to four times) by augmenting ventilatory stroke volume; breathing frequency remained constant. O2 added to the inspired water in maintained hypercapnia reduced Vg at all but the highest level of PCO2. 3. Vg increased when blood oxygen content was decreased by exposure to normoxic hypercapnia, but addition of O2 to the water increased blood oxygen content and Vg decreased. 4. When blood oxygen-carrying capacity was depressed by hypoxia or anaemia, Vg increased as it did during normoxic hypercapnia. 5. We suggest that ventilatory responses to low levels of hypercapnia, to hyperoxic hypercapnia, to hypoxia, and to anaemia in trout are related to changes in levels of blood oxygen content under these conditions.

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