Summary.1. Between 0—40 μmoles of inorganic phosphate per ml of extracellular fluid was rate determining for aerobic glycolysis in Ehrlich ascites tumour cells. The oxygen uptake during aerobic glycolysis was either unaffected or slightly inhibited by phosphate.2. Aerobic glycolysis was accompanied by a reduction in cellular inorganic phosphate and an accumulation of total acid soluble organic phosphate, acid labile phosphate, hexosephosphates and ATP. The ATP increased at the expense of ADP and AMP. In IAA inhibited cells, acid soluble phosphate esters accumulated during aerobic glycolysis, but the total adenine nucleotides decreased markedly.3. Glycolysis as well as respiration greatly stimulated the uptake of P32into organic phosphate fractions, while the concentration of inorganic phosphate in the medium seemed to be a major factor determining the total uptake of P32by the ascites tumour cel