When cerebral tissues are incubated in glucosecontaining media, addition of glutathione increases their aerobic glycolysis. This has led to suggestions that glutathione plays a part in the Pasteur effect (Geiger, 1935; Baker, 1937; Weil-Malherbe, 1938). The tissues' respiration and glycolysis in glucosecontaining media are increased also by electrical pulses and by 20-100 mM-potassium salts, and these increases are smaller in the presence of cysteine (Mcllwain, 1954, and unpublished work). Cerebral respiration and glycolysis in vivo and in ordinary glucose media normally proceed at only