WE have recently reported1 that the oxidation of pyruvate by finely ground preparations of pigeon's brain is catalysed by fumarate and adenylic acid (or adenosine triphosphate). The effect of fumarate can be obtained in non-dialysed preparations, that of adenylic acid after short dialysis (two hours), when the activity of the system can be fully restored to the value before dialysis. Inorganic phosphate is another component of the system (Table 1). After two hours dialysis the preparations still contain some phosphate (about 0·001 m.) and certainly some fumarate2, hence the oxygen uptake in the absence of added phosphate or fumarate is slightly higher than in the controls without pyruvate. These facts have now been confirmed with preparations from rabbit's brain.