The role of a cholinergic muscarinic inhibitory mechanism in sympathetic neurotransmission was investigated in isolated rabbit and guinea pig hearts with intact sympathetic nerves. The effect of varying frequencies of stimulation (2.5, 5 and 10 Hz) on the concentration of noradrenaline (NA) and dopamine-beta-hydroxylase (DBH) released into the perfusate was investigated. Stimulation in the presence of atropine sulfate (3.4 micrometer) resulted in an augmented outflow of NA at all three frequencies while DBH outflow was significantly incrreased only at 5 and 10 Hz. d-Tubocurarine (2.0 micrometer) attenuated the augmenting effect of atropine on NA release at all frequencies of stimulation whereas it negated the significant effect of atropine on DBH release. Nerve stimulation in the presence of acetylcholine (0.55 micrometer) resulted in a significant decrease in the concentrations of NA and DBH in the perfusate. It is suggested that atropine augments NA outflow in part by blocking an "intrinsic" muscarinic inhibitory mechanism. Acetylcholine's inhibitory effect on NA release is reflected in a similar decrease in DBH release and, therefore, may function in vivo via an effect on exocytosis at the adrenergic nerve ending.