Ambulatory intra-arterial blood pressure recording provides a precise and objective method for the study of antihypertensive drugs and their effects on blood pressure. Application of this technique has shown that β-adrenoreceptor-blocking drugs are only effective during the day when the subject is active. They have little effect on the early-morning rise in blood pressure, which is the time of greatest sympathetic nervous activity. Addition of a diuretic to the beta blocker produces a uniform reduction in blood pressure, which is sustained throughout the 24-hour cycle. Carvedilol is a beta blocker with alpha-blocking activity and its profile of action is similar to that of a combination of a beta blocker and a diuretic. The possession of both properties in a single molecule is advantageous.