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Measurement of serum-phenytoin concentrations in 17 elderly patients showed that levels of 40 mumol/l or more were not produced by daily doses of less than 300 mg/day, but that doses of 350 mg/day or more produced higher and potentially toxic levels. Determination of mean values for Vmax and Km for phenytoin gave values little different from those for younger patients. The implications of these findings for anticonvulsant therapy in the elderly are discussed.