Heart rates of 5 rhesus monkeys (Macaca mulatta) were monitored during classical conditioning trials consisting of a visual conditioned stimulus followed after 10 sec by an electric shock to the tail. Heart rates typically increased at the onset of the visual stimulus, and returned to baseline before shock delivery. Autonomic blocking agents were subsequently administered; their effects on resting heart rates, and on acceleratory and deceleratory phases of the biphasic conditioned heart rate responses were examined, both in the raw data, and with a statistical regression technique. Beta-adrenergic blockade by propranolol lowered resting heart rates and was found, after regression analysis, to reduce the heart rate increase phase, and to weakly enhance the subsequent heart rate decrease phase of the conditioned response. Vagal blockade by atropine sulfate elevated resting heart rate, and markedly reduced both acceleratory and deceleratory heart rate phases of the conditioned responses. Ganglionic blockade by chlorisondamine also elevated resting heart rates (less than atropine), and almost completely eliminated conditioned heart rate changes. Several sources of evidence suggest a predominant vagal tone over resting heart rates, as well as mostly vagal mediation (with some sympathetic contribution) of the biphasic conditioned rate response.