Oral medication with phentolamine SR produced subjective and objective improvement in all 45 patients with refractory chronic heart failure of various aetiologies, who were already receiving digitalis and diuretics. This was shown by an increase in pulse-pressure amplitude and urine volume, a decrease in bodyweight and marked reduction in cardiac size and pulmonary congestion, the latter changes being more pronounced in patients with arteriosclerotic heart disease (ASHD). Exercise tolerance tests in a further 34 patients with less severe chronic heart failure demonstrated that phentolamine SR markedly increased physical capacity. This effect was more pronounced in patients with slightly compromised cardiac status (functional Class 2, NYHA) than in those with moderately compromised cardiac status (Class 3). The improvement in clinical condition was attributed mainly to arterial and venous dilatation, as well as to the positive inotropic effect of the drug. The most important side-effects were diarrhoea and, in the patients with ASHD, increased attacks of angina pectoris; The side-effects were well controlled by the anticholinergic agent oxyphenonium bromide (Antrenyl) and a slight increase in the dose of nitrates. It is concluded that oral phentolamine SR may be a valuable therapeutic adjunct in the management of patients with refractory chronic heart failure.