1. 1. Phosphorus nuclear magnetic resonance (31P NMR) provides a non-destructive method for the quantitative observation of the most abundant phosphate-containing metabolites in isolated perfused kidney and in kidneys inside an anaesthetised animal. 2. 2. In this paper, the NMR method is introduced and the intracellular pH changes following acute renal acidosis are measured (from the resonance position of Pi) for the isolated perfused rat kidney. 3. 3. The results are discussed in relation to the biochemical data obtained by freeze extraction.