The reactivity of the sympathetic nervous system was studied in 10 young, adult patients with labile hypertension and compared with a normotensive age-matched control group. Upon graded exercise on a constant speed bicycle ergometer, the hypertensive subjects reacted with an exaggerated blood pressure response and a significantly greater increase in the plasma adrenaline and noradrenaline levels. The basal catecholamine levels, however, were similar in both observation groups. This was in spite of an intact baroreceptor reflex in the hypertensives as indicated by a normal hemodynamic response to angiotensin II. This apparent discrepancy may be explained by an enhanced uptake of adrenaline during stress into the neuron, where it acts as a cotransmitter and facilitates the release of noradrenaline via presynaptic beta 2-adrenoceptors. Similar blood pressure and heart rate responses to isoproterenol and atropine were observed in both groups. This indicates normal beta-adrenoceptor sensitivity and vagal nerve activity in the hypertensive subjects.