Several opportunities for improving animal efficiency through manipulation of metabolism are discussed. The first opportunity is through identification and selection of animals achieving close to theoretical efficiencies. Based upon differences between highly efficient and average animals, the estimated opportunity for improvement is 20%. A second opportunity for improvement is through manipulation of apparent maintenance requirements. Several contributors to differences in efficiencies are considered. One is the contribution of differences in relative organ weights to differences in apparent maintenance requirements. A potential benefit in the order of 10 to 20% through selection or manipulation seems possible. Manipulations of ion transport and protein turnover could yield maximum benefits of 30 and 15%, respectively. However, complete elimination of these processes is not feasible. Without ion transport, membrane potentials would not be maintained and, without turnover, many important regulatory processes would be affected. The limit to manipulation of these characteristics is unknown. A third opportunity for improvement of animal efficiency is through improvement of apparent biosynthetic efficiency by manipulation of patterns of nutrient utilization. If we could produce, through hormonal or other types of manipulations, an optimum pattern of nutrient use, decreases in heat increments of production in growing animals in the order of 50% might be achieved.