A case history is presented of a patient who developed a severe phenytoin intoxication on a "therapeutic" dose of 300 mg/day. This phenomenon could be ascribed to a poor oxidative metabolizing capacity of this patient for phenytoin, as demonstrated by a low para-hydroxyphenyl-phenylhydantoin to phenytoin ratio in the urine. To characterize the specificity of this metabolic defect, debrisoquine and antipyrine oxidation were also studied. Contrary to expectations, this patient was shown to be an extensive debrisoquine metabolizer; the antipyrine clearance was even higher than normal. These findings suggest that phenytoin para-hydroxylation is regulated by an oxidative enzyme complex different from those which oxidize debrisoquine and antipyrine.