SYNOPSIS. Aquatic habitats, especially those of the intertidal, are subject to massive fluctuation of O2 level including periods of both extreme hypoxia and hyperoxia. Most animals have some ability to regulate oxygen consumption during oxygen depletion, although the critical oxygen level above which compensation is possible varies complexly with many external and internal factors. Mechanisms of detection of, and compensation for, hypoxic exposure are examined, in both the short and long-term. The time courses of behavioral, ventilatory and circulatory responses to hypoxia are outlined and the limitations of their role in maintenance of aerobic metabolism are discussed. The role of adjustment of O2 binding properties of respiratory pigments in maintenance of O2 consumption in hypoxia is also explored. Finally the role of anaerobic energy production is briefly outlined.