Fit volunteers were given three doses of 1.5 μg./kg. of the bases of scopolamine or atropine intravenously over a period of 20 minutes. Heart rates fell at first, then rose greater after scopolamine than atropine. Within 30 minutes after the third dose of scopolamine, a striking secondary bradycardia developed, which lasted for three hours. Secondary bradycardia was not seen after atropine. In one experiment with scopolamine, cardiac output was measured and peripheral vascular resistance was calculated. Output varied directly, and resistance inversely in proportion to heart rate.