Abstract 1. 1. Diphenylhydantoin was administered intravenously to 57 patients in whom 73 cardiac arrhythmias occurred. Thirty-one of 49 arrhythmias in patients on digitalis preparations were converted by diphenylhydantoin. Of 24 arrhythmias in patients not on digitalis, diphenylhydantoin was effective in 1. All responses occurred within five minutes after drug infusion. 2. 2. In digitalized patients, conversion of an arrhythmia to regular sinus rhythm or significant slowing of ventricular response following diphenylhydantoin may be considered supportive evidence that the arrhythmia was digitalisinduced. However, unresponsiveness to diphenylhydantoin in a digitalized patient should not necessarily mean that the patient is inadequately digitalized. 3. 3. Minor side effects included transient pain at the infusion site and light-headedness. Three patients experienced profound hypotension of brief duration (2 required metaraminol), and 1 experienced sinus arrest and asystole of six seconds' duration prior to conversion of his arrhythmia. None of these episodes was associated with serious sequelae. The occurrence of hypotension indicates the need for slow drug administration. 4. 4. Diphenylhydantoin should not be used in patients who have a high degree of heart block or marked bradycardia.