Neurohumoral mechanisms operating via the catecholamines are discussed in their relationship to such hypertensive diseases as pheochromocytoma and labile and established essential hypertension. 2. In pheochromocytoma, diagnosis depends almost entirely on identification of increased amounts of catecholamine metabolites in the urine. Because of the danger, manipulative or invasive procedures both for diagnosis and during surgery should be kept at a minimum. 3. In established essential hypertension, reactivity to norepinephrine and plasma norepinephrine are increased, whereas norepinephrine uptake and apparent secretion rate are decreased. 4. In labile essential hypertension, reactivity to epinephrine and probably plasma epinephrine are increased and uptake of epinephrine decreased. 5. Labile hypertension with all its characteristics may or may not coexist with established essential hypertension with all its features. 6. The sympathetic nervous system is also involved in other types of hypertensive disease. Many patients with renovascular hypertension as well as with primary and secondary hyperaldosteronism also have essential hypertension. Angiotensin II affects the sympathetic nervous system and the juxtaglomerular apparatus appears to be beta adrenergic receptor activated, at least in part.