A method for quantitation of in vivo 31P metabolite concentrations in human brain with 31P magnetic resonance spectroscopic imaging (MRSI) is described. The method relies on comparison of brain and calibration phantom measurements, with corrections for coil loading and metabolite magnetic relaxation. Estimated metabolite concentrations for the centrum semiovale in 11 normal adults (mean +/- SD) were: phosphomonoesters = 3.0 +/- 0.7 mM, inorganic phosphate = 0.7 +/- 0.2 mM, phosphodiesters = 10.9 +/- 1.8 mM, phosphocreatine = 2.7 +/- 0.5 mM, and adenosine triphosphate = 2.9 +/- 0.3 mM. These values are similar to previous results obtained from single-volume localized spectroscopy.