It is proposed that the purine nucleotide cycle and glutamine oxidation play a key role in the adaptation of tumour energetics to the transition from the anaerobic to the aerobic state. In support of this proposal, it was found that glutamine and inosine markedly increase total adenylates in the presence of oxygen, whereas the addition of hadacidin abolishes this effect. Transition of the cells from the anaerobic to the aerobic state, and vice versa, in the presence of glutamine plus inosine revealed that there are two components of the adenine nucleotide pool, one which is stable and the other which is variable and responds to the aerobic-anaerobic transition. This part of the pool undergoes degradation or resynthesis owing to activation of the enzymes of the purine nucleotide cycle. Resynthesis of the pool is accompanied by substantial net utilization of aspartate, which is produced by glutamine oxidation. This is supported by the experiments in which the cells were alternately incubated with nitrogen or oxygen, demonstrating that hadacidin significantly decreased utilization of aspartate and regeneration of ATP owing to inhibition of adenylosuccinate synthase.