There seems to be little doubt, when the general field of metabolic inhibition is contemplated, that the role of metabolic inhibitors in the direct control of disease is undergoing serious assessment at the present time. The manner in whitch inhibitors act, a firm knowledge of the underlying principles concerning their activities, will provide the means in the future for the development of drugs with greater potency and specieficity. The recognition of the importance of feedback mechanisms, whereby products of enzymic reactions the rates of their own synthesis, or of cases where a metabolite of one enzymic reaction sequence is also a competitive inhibitor od another, separete but related, sequence is an important step forward our understanding of cellular reaction processees. An appreciation of the properties of methabolic inhibitors drawn from a great variety of enzyme studies is vital for work which is expected to lead to further understanding of abnormal cell growth, for the development of antitumor agents, for obtaining knowledge of genetic relationships to disease conditions, and for the exploration of processes as distinct from static metabolic states. A relatively new field in the realms of biochemistry and physiology is energing from the recent work on trasport carriers across cell membrannes. Here again, a knowledge of the effects of structural analogues and other types of metabolic inhibitors on the specific processes governing the activity of such carriers is of paramount importance in the development of experimental approaches to problems of this type. This field promisses to be as crucial in future work on the control of cell behavior as knowledge of specific inhibitory phenomena of isolated enzyme systems has been in bringing us to the present stage of understanding.