31P NMR spectra of the rat liver were recorded in vivo at 2.35 and 8.5 T. There was a large peak in the phosphodiester region of spectra obtained at 2.35 T which was much reduced at 8.5 T. The peak at 2.35 T is unlikely to be primarily from free cytosolic phosphodiesters, which would not be expected to display such a marked field dependence.