Simultaneous measurements of DNA cell phase cycle distributions and in vivo 31P NMR spectroscopy were performed on 40 RIF‐1 murine tumors irradiated with 14 Gy of X‐radiation. Diploid and tetraploid tumor populations were observed. The cells blocked in G2/M phase were measured as a function of the ratios of tetraploid cell number in G2/ M phase versus total cell population measured. The G2/M population reached a maximum at 32 h post irradiation, dropping to control values by 72 h, while the ratio of inorganic phosphate to β‐nucleotide triphosphate dropped significantly at 32 h and remained significantly lower than control up to 72 h post irradiation. Measurements of PME, PDE, PCr, and pH showed no significant variations at any time point. No significant change in host cell population could be observed. Since the measured G2/M population never increased to more than 3% of the total cell population, the change observed in the 31P NMR spectra were not simply the result of possible differences in NMR profiles of the different cell phase populations but were more likely due to a change in the metabolic characteristics or environment of a majority of the Cells. © 1990 Academic Press, Inc.