A prejunctional mechanism involving an α1-adrenergic receptor may exert control on the release of acetylcholine from parasympathetic nerve endings in the heart. To test this hypothesis in vivo, rats were prepared for electrical stimulation of the vagus nerves. Blood pressures and heart rates were monitored, and the animals were treated with α-agonists and α-antagonists. The α1-selective agonist phenylephrine significantly attenuated vagally induced bradycardia in a dose-dependent fashion (ED50= 19 μg/kg). This is consistent with the hypothesis that there is α-adrenergic inhibition of ACh release. In contrast, the α2-selective agonist, BHT-920, caused no change in heart rate during vagal stimulation. The effects of phenylephrine to raise heart rate and blood pressure during vagal stimulation were blocked by the α1-selective antagonist prazosin (ID50approximately 1 μg/kg) but not by the α2-selective antagonists yohimbine and rauwolscine. This further supports an α1assignment to the prejunctional adrenergic receptor mechanism, which can regulate the release of acetylcholine from cardiac parasympathetic neurons.