The hemodynamic response to emotional stress (mental arithmetic) was studied in ten borderline and ten mild hypertensives in comparison to ten age-matched normotensives. Due to emotional stress the blood pressure increased in all three groups; its rise was greater in borderline and mild hypertension. After stress, the blood pressure also remained elevated longer in the hypertensive groups. The increased blood pressure reaction was accompanied by a greater rise of heart rate and cardiac output. The increase of cardiac output correlated with the blood pressure elevation. Stroke volume changed in comparison to the pre-stress period only in the mild hypertension group. Total peripheral resistance decreased slightly in all three groups. No significant differences between the groups could be found in regard to stroke volume and total peripheral resistance. The cardiovascular hyperreactivity in early hypertension is attributed to a stronger sympathetic stimulation of the circulation. The hemodynamic pattern indicates an increased beta-adrenergic stimulation of the heart as the main pathological mechanism.