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39K nuclear magnetic resonance (NMR) spectra were readily obtained, in vivo, from rat muscle, kidney, and brain in 5–10min with signal-to-noise ratios of approximately 20:1. Quantitation of the K+ signal was achieved by reference to an external standard of KCl/dysprosium nitrate as well as by reference to the proton signal from tissue water. In vitro NMR studies of isolated tissue showed a K+ visibility (NMR K+/total tissue K+) of 96%, 62 +/- 8%, 47 +/- 1.9%, 45 +/- 3.5%, and 43 +/- 2.5% for blood, brain, muscle, kidney, and liver, respectively. Absolute tissue K+ was determined by flame photometry of acid-digested tissue. Changes in tissue K+ status by chronic K+ depletion or acute K+ loading produced changes of 39K NMR signal intensity that were equal to changes of absolute tissue K+. Acidosis, alkalosis, mannitol, or RbCl infusion did not significantly change the NMR K+ signal. These results indicate that the …