1. Slight decreases and increases in carotid baroreceptor activity were induced in subjects with essential hypertension by slight alterations in carotid transmural pressure (variable pressure neck-chamber technique) in order to obtain limited increases and reductions in sympathetic adrenergic activity.
2. When sympathetic activity was reflexly increased there was a rise in arterial pressure but no significant increase in plasma catecholamines. Likewise when sympathetic activity was reflexly reduced there was a fall in arterial pressure but no significant reduction in plasma catecholamines.
3. Plasma noradrenaline and adrenaline significantly and markedly increased in the same subjects when sympathetic activity was increased by activation of both arterial and low pressure baroreceptor reflexes with tilting.
4. It is suggested that measurements of catecholamines in systemic plasma may reveal marked degrees of sympathetic activation but may not be a sensitive index of more moderate changes in sympathetic tone.