In animals, baroreflex sensitivity is inversely related to the likelihood of ventricular fibrillation during myocardial ischemia. After myocardial infarction in human patients, reduced baroreflex sensitivity is associated with increased mortality. A reduced standard deviation of normal RR intervals over a 24 h period is also associated with reduced survival after myocardial infarction. Therefore, 32 normotensive men who had survived their first myocardial infarction were studied to define the relation between baroreflex sensitivity assessed with phenylephrine injection and three Holter electrocardiographic measures of tonic vagal activity: the percent of successive normal RR intervals greater than 50 ms, the root mean square successive difference of normal RR intervals and the power in the high frequency energy of the normal RR interval power spectrum. Correlations among the Holter measures of heart period variability were greater than or equal to 0.94, indicating that these measures are so strongly correlated that any one of them can be used to represent the others. Baroreflex sensitivity showed weaker correlations with the three Holter variables (0.57 to 0.63), indicating that the Holter measures did not accurately predict baroreflex sensitivity. Baroreflex sensitivity showed a stronger correlation with the three Holter variables during the night than during the day. Baroreflex sensitivity and tonic vagal activity reflected by Holter variables were reduced more in patients with inferior myocardial infarction than in those with anterior infarction. The relative utility of baroreflex sensitivity and Holter measures of tonic vagal activity in predicting sudden cardiac death after myocardial infarction needs to be evaluated in a large prospective study.