1. Methods for cannulating the ventral aorta of the trout, which permit the measurement of blood pressure in the unanaesthetized, unrestrained, intact fish are described.

2. Rate and amplitude of breathing and blood pressure in the dorsal and ventral aortae increase during hypoxia. These changes are associated with a marked bradycardia.

3. There are increases in vascular resistance to blood flow in both respiratory and systemic circulations during hypoxia.

4. Post hypoxia is associated with large increases in ventral aortic and dorsal aortic pressures.

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