The intravenous administration of hematin, in a patient with acute intermittent porphyria in relapse, was followed by a rapid decline of porphyrin precursors in the blood and urine, together with complete remission of the porphyric symptoms and manifestations, including abdominal pain, vomiting, constipation, tachycardia, urinary retention, and psychopathic behavior. The hematin was given for 3 days, in four doses of about 300 mg each. By serial spectroscopic observation, hematin was constantly present in the blood and available for perfusion during at least 90 hours, disappearing between 30 and 36 hours after the last unit was given. The relation of the remission to the remarkable lowering of serum and urinary δ-aminolevulinic acid and porphobilinogen and the underlying repression or inhibition, or both, of the induction of aminolevulinic acid synthetase by the hematin is discussed. Further studies to explore this question and provide information on the value of the method in aborting attacks at an early stage are needed.