A technique to determine absolute metabolite concentrations of the kidney in vivo using 31P NMR is described. The technique is based on the use of methylphosphonic acid (MPA), which gives rise to a well-resolved peak upfield from in vivo phosphorous metabolite resonances, as an "internal standard." The method involves acquisition of a fully relaxed kidney spectrum with an implanted coil followed by intravenous infusion of MPA (4 ml of 150 mM) for a period of 30 min. The animal is then sacrificed to insure a steady state level of renal MPA and another spectrum is obtained. From these two spectra the ratio of intensities of MPA to P-ATP was derived. In the method used here, no significant contribution from tissues outside the kidney was observed. In addition, a relatively homogenous distribution of MPA throughout the kidney was achieved. The amount of MPA per gram wet weight of kidney was also obtained through NMR methods by placing the excised organ in a phosphate-calibrated solenoidal coil. The calibration spectra along with the ratio of intensities for MPA/@-ATP were used to calculate the number of micromoles of ATP per gram wet weight of kidney. Infusion of a higher concentration of MPA (1.25 M ) produced a visible MPA resonance in other organs besides the kidney. Thus, MPA could be useful in determining phosphate metabolite concentrations in other tissues. o 1987 Academic Press, Inc.