Dose-response curves for the effect of atropine sulfate on heart rate were determined in six hyperthyroid and six euthyroid subjects. Both groups responded to atropine in a qualitatively similar manner, but a smaller increase in heart rate was produced by atropine in the hyperthyroid group. The data are interpreted as suggesting that the resting tachycardia of hyperthyroidism is due to the combination of a direct rate-increasing effect of thyroxine on the sinoatrial (SA) node and an impairment of vagal inhibition of heart rate.