Abstract
A standard step test was applied to 13 ambulant fibrillators. Increase in heart rate exceeded that of normals following exercise. No correlation was found between any one of the usual indices and the clinical state. Response in a given patient was relatively constant. In some a brief fall of rate occurred at the beginning of exercise. The most constant feature was a delayed acceleration commencing about 12 sec. after the exercise started. Digitalis reduced but did not eliminate the very marked tachycardia following exercise.