ROSENTHAL1 has shown that when the anaerobic conversion of fructose to lactic acid by the Jensen rat sarcoma is studied by Warburg's method, a spontaneous increase in the rate of acid production , which he ascribes to the anaerobic formation of an activator, occurs about forty minutes after the manometric vessels are put into the thermostat at 38° C. We have been able to show that this spontaneous increase may also be demonstrated when glucose is the substrate, although here the pre-activation period is shorter; and we find that with both sugars the pre-activation period is abolished when sodium pyruvate is present in 10−3 M concentration.