Rate of oxygen consumption, critical oxygen tension, capacity for anaerobiosis, and depletion of oxygen within the mantle cavity have been determined for high- and low-level populations of Mytilus californianus Conrad. High-level mussels had a significantly higher rate of respiration when immersed at 10°C, compared with low-level animals. Following exposure periods of 6 and 12 h, there was a rise in oxygen consumption which was greatest in the high-level mussels: the extent of the rise in metabolic rate was a function of time of exposure only in the high-level animals. Both the high- and low-level mussels are metabolic regulators, with the high-level animals showing a critical oxygen tension value slightly lower than the low-level animals. The levels of tissue lactate were initially low, and the accumulation was not significant after 12 h exposure. There was, however, a tendency for the high-level mussels to accumulate more lactate in the tissues than the low-level animals. Oxygen in the mantle cavity fluid decreased exponentially with time of exposure, although the initial decrease was less in the high level-forms. Field measurements deviated from those obtained on 'restrained' laboratory mussels, and the difference is discussed in terms of the possible utilization of atmospheric oxygen. It was concluded that one consequence of high-tide existence is an increase in metabolic rate during immersion and a greater capacity for anaerobiosis during exposure in comparison with the low-level mussels.