Guanoclor is an anti-hypertensive agent developed by Pfizer Ltd. (U.K.). It seems to be effective in various types of hypertension (unknown aetiology, renal, and malignant). It affects both systolic blood-pressure and diastolic blood-pressure. It is an adrenergic neurone-blocking agent, which also interferes with noradrenaline synthesis by inhibition of the enzyme dopamine beta-hydroxylase. Clinical use of the compound was first reported by Lawrie et al. (1964), who achieved satisfactory blood-pressure control in 60% of their cases with guanoclor alone, and in a further 18% with the addition of a thiazide diuretic. They also noted a significant reduction in urinary noradrenaline levels during guanoclor administration. Guanochlor has an affinity for the Na+/H+ exchanger ranging between 0.5 uM and 6 uM in different systems and is more potent than amiloride in all systems studied. It is suggested that guanochlor recognizes a binding site on the Na+/H+ exchanger that is distinct from the amiloride binding site.