Some disorders in which excessive sweating, hyperhidrosis, is a symptom are also characterized by increased sympatho-adrenal activity. Such disorders are hypotension, hypoglycemia, pheochromocytoma and hyperthyroidism. Sweat glands are controlled by a cholinergic innervation but can also be stimulated by adrenergic agents whose effects can be blocked by both alpha- and beta-receptor blocking drugs. An adrenergic innervation has also been demonstrated. There is evidence that the adrenergic component of sweating particularly secretion of the adrenal medulla, is responsible for the enhancement of sweating during exercise but not for the hyperhidrosis present in these disorders since sweating in these circumstances can be effectively blocked by cholinergic blocking compounds. Cutaneous vasoconstriction due to elevated plasma catecholamines reduces the rate of evaporation of sweat and allows sweat to accumulate on the skin. It is suggested that in the case of hypotension and insulin hypoglycemia sweating results from general sympathetic stimulation and that adrenal medullary hormones are not an essential component of the response. Hyperhidrosis in pheochromocytoma may be due to central activation of heat loss mechanisms resulting from the passage of plasma catecholamines across the blood-brain barrier combined with increased thermogenesis and cutaneous vasoconstriction. The hyperhidrosis of hyperthyroidism is probably due to increased thermogenesis.