Summary.I. In the last few years it has been found that:(a) A large proportion of the acid‐soluble phosphate originally classed as “inorganic” is actually combined with creatine (creatinephosphoric acid, phosphocreatine, phosphagen).(b) Most of the organic phosphate hydrolysed by the muscle enzymes during autolysis (the fraction hitherto known as “lactacidogen”) is adenosinetriphosphoric acid (adenylpyrophosphate).(c) There is no hexosediphosphoric ester in normal muscles, but a small amount of a hexosemonophosphoric ester. To this the name lactacidogen is now applied.2. Concerning the chemical events accompanying activity of a muscle, two discoveries in particular have greatly altered their interpretation:(a) It is found that the production of lactic acid occurs partly, and in some cases entirely, after the activity has ceased.(b) The production of lactic acid does not occur at all in muscles suitably poisoned with fluoride or iodoacetate. In such muscles the energy production is proportional to the extent of the accompanying phosphagen breakdown and is limited by the amount of phosphagen in reserve.3. Arising out of these and other considerations discussed in the text, a reasonable working hypothesis of muscular contraction is that the thermal and mechanical energy released in activity comes from the (exothermic) breakdown of phosphagen During the subsequent recovery period this phosphagen is resynthesised, the necessary energy being derived:(a) In the absence of oxygen; from the conversion of glycogen to lactic acid. Restitution in this case is only complete in muscles which have used up nearly all their reserve of phosphagen. There seems to be no alternative to the glycolysis mechanism as an anaerobic source of energy.(b) In the presence of oxygen, from the combustion probably of any available foodstuff, though carbohydrate seems to be the material used for choice by skeletal muscles.4. Other directions in which rapid advance has been made in recent years are:(a) The isolation of the constituent parts of the ferment system responsible for the glycolytic process in muscle.(b) The perfection of a delicate and rapid method of measuring the vapour pressure of muscles. One of the immediate results of the application of this technique has been the demonstration that the catabolic changes so far studied in connection with muscular activity do not account for all the osmotic pressure change. There must remain reactions of quite considerable extent so far und