Abstract A method for measuring potassium is described which is based on the requirement of pyruvate kinase for this cation. Activity of this enzyme is linear with respect to potassium concentration between 10 and 100 μ m. The rate of pyruvate formation is measured with lactic dehydrogenase and NADH in a coupled assay. Other common cations do not interfere at concentrations likely to be present. With a variety of plant extracts, potassium concentrations obtained with the assay were in agreement with those obtained using a flame photometer. For measuring smaller amounts of potassium the reaction is carried out in a small volume and the NAD + produced is converted to a highly fluorescent product or is amplified by enzymatic cycling. These procedures have been tested extensively only on plant material, but the assay should be generally applicable.