Summary.1. The aerobic inhibition of glucose utilizationi. e.the Pasteur effect in Ehrlich ascites tumour cells decreased when the concentration of inorganic phosphate in the medium and the level of intracellular inorganic phosphate increased. It was accompanied by a rise in acid soluble phosphate esters. A higher aerobic than anaerobic glucose uptake was observed in IAA inhibited cells, or by incubation in a strong phosphate buffer (negative Pasteur effect).2. In ascites tumour cells, partially blocked with malonate, fumarate in contrast to citrate, stimulated oxygen uptake and inhibited glycolysis (the malonate Pasteur effect). This effect also decreased when the concentration of inorganic phosphate in the medium and the level of intracellular, inorganic phosphate increased. It was inhibited by DNP and anaerobiosis.3. In glycolyzing cells, blocked with malonate, fumarate further enhanced the acid labile phosphate and ATP, in spite of the accompanying rise in oxygen uptake. Thus the Crabtree effect could not be explained by lack of total cellular inorganic phosphate or phosphate acceptors.4. The Pasteur and Crabtree effects in Ehrlich ascites tumour cells are explained by the preferential channeling of limited a‐mounts of cellular inorganic phosphate which is to some extent carried by adenine nucleotides into certain reactions at the expense of other