THE quantitative yield of phosphate fixed during cellular oxidations deserves critical investigation, for it determines the efficiency with which utilizable energy may be derived from assimilated foodstuffs and it may influence metabolic regulatory mechanisms1. This problem has been the subject of many investigations (cf. Krebs et al. 2 andSlater and Holton3). In general, the observed yield has increased over a period of years as newer methods of measurement have been developed and particularly as more knowledge has been made available concerning the preparation of undamaged mitochondria which are free from interfering cellular components.