SUMMARY1. Both dietary magnesium depletion and potassium depletion (confirmed by tissue analysis) were induced in rats which were then compared with rats treated with chlorothiazide (250 mg/kg diet) and rats on a control synthetic diet.2. Brain and muscle intracellular pH was measured by using a surface coil and [31P]‐NMR to measure the chemical shift of inorganic phosphate. pH was also measured in isolated perfused hearts from control and magnesium‐deficient rats. Intracellular magnesium status was assessed by measuring the chemical shift of β‐ATP in brain.3. There was no evidence for magnesium deficiency in the chlorothiazide‐treated rats on tissue analysis or on chemical shift of β‐ATP in brain. Both magnesium and potassium deficiency, but not chlorothiazide treatment, were associated with an extracellular alkalosis.4. Magnesium deficiency led to an intracellular alkalosis in brain, muscle and heart. Chlorothiazide treatment led to an alkalosis in brain. Potassium deficiency was associated with a normal intracellular pH in brain and muscle.5. Magnesium depletion and chlorothiazide treatment produce intracellular alkalosis by unknown m