We consider stress as an environmental change that results in reduction of net energy balance (i.e. growth and reproduction). Reduced energy balance restricts the environmental range of an organism and may change the environmental optima at which maximum production can be achieved. We emphasize individual differences in net energy balance and the interrelationships among genetic heterozygosity, rate of protein synthesis, efficiency of protein synthesis and whole organism measures of both routine and maintenance metabolic rate. Lastly, we consider the consequences of genetically determined individual differences in metabolic maintenance costs within the context of variable environments and how genetic/environmental interactions can define individual responses to environmental extremes.