The effects of intracardiac infusions of adenosine on the changes in heart rate (HR), electrocardiogram (ECG) and arterial blood pressure (BP) induced by both vagal stimulation and exogenous acetylcholine (ACh) were studied in anesthetized rats. Adenosine inhibited the bradycardia induced by vagal nerve stimulation, an effect antagonized by theophylline, decreased the elongation caused by vagal stimulation of the R-R intervals of the ECG, and caused a small but consistent decrease in the hypotensive effect of vagus nerve stimulation. At the dose that reduced the bradycardiac responses to vagal stimulation, adenosine enhanced the bradycardiac effect of exogenous ACh, increased R-R intervals and the number of P waves not followed by the ECG and had little or no effect on the inhibition induced by ACh on BP. The effects of adenosine on the bradycardiac responses to vagal nerve stimulation or to ACh administration were similar in both non-reserpinized and reserpinized animals. These results suggest that exogenous adenosine can modify the vagal influences on the heart by exerting pre-junctional inhibition of the vagus nerve and post-junctional enhancement of the ACh actions, and that the adrenergic system does not contribute to these effects of adenosine.