I. Blood samples were taken during prolonged hypoxia experiments in which the inspired water oxygen tension was less than 10 mmHg. The oxygen tension of the post-branchial blood was about 5 mmHg and its pH shows a significant lowering from normoxic levels. 2. The decrease in blood pH is correlated with increases in levels of lactate and pyruvate. The lactate/pyruvate ratio increases during hypoxia. 3. An increase in blood succinate was also found, and strongly suggests the accumulation of multiple anaerobic end-products within the tissues. 4. Recovery of normoxic levels of succinate takes place almost immediately following the restart of ventilation whereas the decrease in lactate concentration is slower. 5. It is concluded that these adaptations may be related to the habitat of the fish at low tide in pools where the Po2 may fall very markedly.

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