Past evidence derived both from reports in the literature and from the Investigators' observations have shown that hypothyroidism and hyperparathyroidism may be associated with hypertension in man, which usually disappears with correction of the thyroid or parathyroid disorder. Since the incidences of these endocrine causes of hypertension are not known, these will be defined by screening patients with hypertension prospectively over 5 years for demonstrable hypothyroidism and hyperparathyroidism. The mechanism of each type of hypertension will be defined by metabolic studies aimed at determining whether the hypertension is of the high-, normal- or low-renin type, whether it is angiotensin-dependent (reversed by saralasin), whether it is mediated by hyperaldosteronism (associated with hyperaldosteronism unsuppressible with standardized saline infusions and is overcome by spironolactone therapy), whether it is mediated by catecholamine excess (associated with elevated plasma catecholamines and overcome by phentolamine), whether it is hypervolemic and volume dependent (associated with expanded plasma volume and reversed by venesection) and whether it is associated with abnormalities in blood bradykinin level or in urinary prostaglandin E2 excretion.