In rabbits: (1) Intravenous atropine in the dose of 0.002-0.004 mg/kg caused bradycardia, whereas in that exceeding 0.02 mg/kg tachycardia. Transient but alarming degrees of tachycardia could be observed when 2 mg/kg dose was employed. (2) While Arfonad was being infused, the heart rate changed little regardless of the doses of atropine used. (3) During Arfonad drip, changes in heart rate seemed fairly well correlated with the degree of hypotension, i.e., magnitude of ganglionic blookade. (4) The above findings suggest that, at least in rabbits, small dose atropine-induced cardiac slowing is due to the inhibitory action of atropine on the sympathetic ganglia. This cannot, however, be directly applied to human physiology because of the possible species difference in the regulatory mechanisms of heart rate.