The effects of intrinsic (body size, gill surface area and activity patterns) and extrinsic (temperature, salinity and oxygen concentration and food) factors on oxygen consumption were measured in the mactrid clam Mulinia lateralis. Both acclimated and acute rates of VO₂ were measured under 9 salinity-temperature combinations and a model is presented which indicates that: 1) body size is the over-riding factor determining VO₂, 2) acclimation temperature alone has no significant effect on either fed or starved animals 3) the interaction of acclimation temperature and salinity and exposure salinity is the strongest factor influencing VO₂ after body size, 4) exposure conditions are more likely to determine energy levels of expenditure than acclimation conditions. Small individuals have a greater gill surface area per body weight than larger individuals. The degree of oxygen independence is dependent on both body size and on acclimation temperature and shows a slight increase after exposure to anoxic conditions. Patterns of shell valve activity were altered and VO₂ increased in response to algae, charcoal and algal extract. There was no evidence of a complete temperature acclimation even after 3 weeks. The scope for activity indicates that starved animals at high temperatures are at a greater energetic disadvantage than individuals at lower temperatures. Equations are presented for clearance/pumping rate, convection requirements and utilization efficiencies and indicate that metabolism increases faster in relation to body size than does pumping rate resulting in decreased net energy gain as the animals increase in size. Survival of M. lateralis populations appears to be a combination of behavioural and physiological adaptations which allow successful exploitation of favourable conditions while at the same time leaving the animals vulnerable to long term stress conditions.

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