1. The elevated beta-receptor responsiveness to adrenergic stimuli makes subjects with the primary hyperkinetic cardiac syndrome ideal for studying the electrical and dynamic responses of the heart to sympathetic activation.

2. In twelve men presenting with the syndrome, the effects of mental arithmetic and painful (cold) stress on the cardiac inotropic state were tested and correlated with the concomitant electrocardiographic changes.

3. Arithmetic and cold evoked responses opposite and divergent from the base-line state: The former induced vasodilatation, enhancement of cardiac rate, output, contractility and deep T wave inversion; the latter caused vasoconstriction, cardiac depression and full restoration of repolarization.

4. The sympathetic outflow elicited by stress is not generalized, but selectively directed to different circulatory levels in relation to the stimulus at work; cardiac sympathetic stimulation or inhibition has opposite effects on the repolarization phase.

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