The effects of hypothermia on the intracellular pH of human erythrocytes were studied non-invasively using 31P NMR spectroscopy and the endogenous phosphorus-containing compounds glycerate 2,3-bisphosphate and inorganic phosphate. Specifically, the pH dependence of the 31P NMR chemical shifts of these compounds was used to measure the intracellular pH at 25 and 37 degrees C. The possibility of a non-pH-dependent change on the chemical shifts of the 2-P and 3-P resonances of glycerate 2,3-bisphosphate due to the presence of paramagnetic deoxy-haemoglobin (i.e., a pseudo-contact interaction) was investigated and found to have negligible effect under the present experimental conditions. The most probable reasons for this are that the deoxy-haemoglobin concentration was too small and/or the glycerate 2,3-bisphosphate does not get sufficiently close to the paramagnetic centre to be affected. The change in intracellular pH with temperature was consistent with that predicted by the alphastat hypothesis.