The pool sizes of purine nucleotides, nucleosides, and nucleobases were measured in the host tissues liver, skeletal muscle, and blood of Ehrlich ascites tumor-bearing mice during the different periods of tumor growth. There were large differences of tissue concentrations of these metabolites between control animals, animals in the logarithmic growth period of the Ehrlich ascites tumor, and animals in the resting phase of tumor growth. The ATP concentrations in liver, muscle, and erythrocytes were higher during the proliferating phase of the tumor compared with the ATP levels of these organs in healthy animals. In liver and skeletal muscle the ATP concentration decreased during the transition from proliferating into resting phase of tumor growth. The concentrations of nucleosides and nucleobases within the RBC and blood plasma deceased during the logarithmic growth phase but restored during the plateau period. As well as in the organs/cells investigated and in the body fluids (plasma, ascites fluid) a tremendous increase of adenosine concentration during the resting phase of tumor growth was observed. From changes of total purine ribonucleotide pattern an activation period in the nucleotide metabolic pathways of liver and skeletal muscle during the proliferating phase of tumor growth is postulated.