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The principles and main features of enzymatic methods for the measurement of sodium, potassium, and chloride are reviewed and their performance compared with current procedures. Each method makes use of a relatively specific enzyme, catalysing a reaction whose rate is sensitive to the ion to be determined. Where the (S)0.5 of the enzyme for the ion is much lower than the assay concentration, the ion concentration may be reduced by a binding agent. Alternatively, a competitive inhibitor may be used to raise the (S)0.5 of the enzyme. In the case of chloride determination with amylase the (S)0.5 of the enzyme is raised by limiting the concentration of free calcium. In the measurement of potassium, interfering ions such as sodium are removed by binding with Kryptofix 221 and improvement in performance is also achieved by use of a bacterial pyruvate kinase less sensitive to sodium. The enzymatic methods are applicable to measurement of sodium, potassium and chloride in blood or urine with good precision, accuracy, and specificity. They can be used on mechanized or manual instruments. There appears to be minimal interferences from compounds found in normal or pathological serum or urine.