1. [31P]NMR spectra were obtained from a functioning isolated perfused rat kidney with the aim of determining intrarenal pH in acute acidosis. 2. Signals from intracellular inorganic phosphate could be observed in the absence of phosphate in the perfusion medium. Under these conditions renal ATP and inorganic phosphate content fell by 30% but total adenine nucleotide and phosphorylation potential ATP/ADP x Pi were unchanged compared with kidneys perfused with phosphate‐containing medium. In addition, G.F.R., Na+ reabsorption and ammonia formation from glutamine remained normal. Ammonia production increased 93%, urine pH fell to 5.8 +/‐ 0.1 and kidney 2‐oxoglutarate content fell by 80% upon acidification of the perfusion medium from pH 7.4 to pH 6.9, findings identical with those obtained in controls (Ross & Tannen, 1979). 3. [31P]NMR spectra of the isolated perfused rat kidney showed a pattern of adenine nucleotides and a small concentration of phosphocreatine, Intra‐renal pH was measured from the resonance position of intracellular inorganic phosphate and in perfusions with pH 7.4 buffer was 7.19 +/‐ 0.10 (n = 11). 4. Acidification of the perfusion medium to pH 7.0 resulted in 0.3 pH unit fall in intrarenal pH. This fall in total intrarenal pH is insufficient to explain the fall in 2‐oxoglutarate concentration observed if the glutamate‐dehydrogenase‐equilibrium model is invoked. 5. The line‐width of the NMR signal is compatible either with heterogeneity of intra‐renal pH or the existence of a pH gradient between cytosol and mitochondria, or both.