Secondary productivity is one portion of energy flow in a community, which includes the ingestion and assimilation of energy, and the expenditure of energy in metabolism by consumer organisms. The purpose of this paper is to describe the ecological significance of energy flow in consumers and to discuss methods of measuring its components. Data pertaining to 20 terrestrial animal populations are presented. About 20% of the energy assimilated by invertebrates is manifested as net production, while only about 2% of the assimilated energy is represented by net production in populations of birds and mammals. The relationship between production and metabolism appears to depend on the capacity for homeothermy. For a given amount of assimilated energy, homeotherms produce less than heterotherms.