M-7, in doses of 1 mug/kg, reduces the resting heart rate of anesthetized dogs and cats. In similar dose ranges, M-7 blocks reflex activation of the sympathetic nervous system. Experimental procedures included bilateral carotid occlusion in the dog and central stimulation of vagi and right sciatic nerves in the cat. M-7 inhibits the response to postganglionic stimulation of the right cardioaccelerator nerve if the frequency of stimulation is 2 Hz; no inhibition is observed with a frequency of 18Hz. The inhibitory action of M-7 is antagonized by haloperidol, 50 mug/kg, or chlorpromazine, 150 mug/kg. Similar inhibitory actions and frequency specificity were observed in vitro using the right atrium of the cat and nerve stimulation. In cats, nerve stimulation was inhibited by apomorphine. Dopamine, in the presence of cocaine, also inhibited low-frequency nerve stimulation. It is postulated that prejunctional sympathetic nerves innervating the heart have dopamine receptors that serve as inhibitory role in transmission.