Seven patients with severe chronic congestive cardiac failure secondary to ischaemic heart disease performed submaximal supine exercise before and after 5 mg sublingual isosorbide dinitrate at the time of cardiac catheterisation. Exercise before isosorbide dinitrate produced a poor response in left ventricular performance. After isosorbide dinitrate this response was significantly improved. Compared with the control exercise period, cardiac index increased from mean 2.6 to 3.1 1/min per m2 (P less than 0.0025), stroke volume index from mean 22 to 27 ml/m2 (P less than 0.0025), and left ventricular stroke work index from mean 21 to 30 g m/m2 (P less than 0.01). Mean left ventricular filling pressure fell from 37 to 26 mmHg (P less than 0.01). Although isosorbide dinitrate reduced left ventricular filling pressure at rest from mean 26 to 17 mmHg (P less than 0.005), there was no significant change in mean cardiac index or stroke volume index, while left ventricular stroke work index decreased from mean 29 to 22 g m/m2 (P less than 0.05). Isosorbide dinitrate effectively reduces left ventricular filling pressure in the resting patient with congestive cardiac failure but produces a more comprehensive improvement in left ventricular performance during exercise.