1. Swimming performance and oxygen consumption of normoxic (control) and hypoxia-acclimated (P₀₀₂=40 mmHg) rainbow trout, Salmo gairdneri Richardson, were monitored at >145, 60 and 40mmHg.

2. Maximum swimming velocity at 40mmHg was reduced from >54.8cm s^-1 to 41.4cm s¹ in controls and to 40.6 cm s^-1 in hypoxiaacclimated fish.

3. Normoxic oxygen consumption of control fish ranged from 97.5 mg O₂ kg^-1 h^-1(5.5cm s^-1) to 318.5 mg O₂ kg^-1 h^-1 (54.8 cm s^-1) and did not differ significantly from that of hypoxia-acclimated fish in normoxia.

4. Reduction of ambient P₀₀₂ from normoxia to 60mmHg or 40mmHg did not significantly change oxygen consumption in control animals, although no fish (control or hypoxia acclimated) completed swimming trials at 54.8cm s^-1 in 40mmHg.

5. Oxygen consumption of hypoxia-acclimated fish at 5.5cm s^-1 and 40 mmHg was significantly higher than oxygen uptake in normoxia at the same speed. This relative increase was not maintained, however, as oxygen consumption at higher swimming speeds was similar to that in normoxia.

6. Blood studies showed that hypoxia-acclimated fish had lower ATP concentrations and P₅₀ values. While these factors may increase the blood oxygen loading capacity, the change is apparently not enough markedly to improve swimming performance or oxygen consumption in hypoxia and/or exercise.