Measurements of short and long-term heart rate and heart period variability were compared. The day-to-day reproducibility and time course of change were determined in measures of heart rate variability and heart period variability in patients with myocardial infarction. The predictive accuracy of heart rate variability measured late after myocardial infarction for subsequent mortality and development of malignant ventricular arrhythmias was determined. Heart rate and heart period variability findings after myocardial infarction were compared with those in age and sex-matched normal subjects and with those made in patients with malignant ventricular arrhythmias.