1. Although heart failure is commonly associated with depressed systolic function, there is increasing evidence that impaired diastolic performance is also universally present and might be a key determinant of symptoms, physical capacity and even survival in some subsets of patients. 2. Reduced diastolic distensibility increases cardiac filling pressure not only at rest, but even more during exercise when diastolic filling time is reduced. The increases in filling pressure and diastolic wall stress lead to pulmonary congestion and subendocardial ischaemia, it also triggers myocardial hypertrophy and a detrimental remodelling of the ventricular cavity. Perhaps even more importantly, impaired ventricular distensibility limits the use of the Frank-Starling mechanism, impairing systolic pump function and cardiac output adaptation during exercise. Therapies able to improve the distensibility of the ventricle are, therefore, desirable in heart failure. 3. Nitrates, angiotensin converting enzyme (ACE) inhibitors and diuretics may indirectly increase left ventricular chamber compliance by their effects on the right side of the heart. Cardiac glycosides do not improve myocardial relaxation and may even cause diastolic contracture at toxic doses. The new beta 1-adrenoceptor partial agonist, xamoterol, on the other hand, consistently lowers left ventricular filling pressure at rest and during exercise, and produces an increase in left ventricular dynamic compliance through the direct lusitropic effect of beta 1-adrenoceptor stimulation. These beneficial effects are maintained during prolonged therapy and also appear sufficient to slow the remodelling of the ventricular cavity. The improvement in symptoms and in exercise tolerance observed during xamoterol (Corwin, Carwin, Corwil, Xamtol, ICI 118,587) therapy might, therefore, be related to the improvement in left ventricular diastolic distensibility induced by this drug.