The effect of chewable isosorbide dinitrate on submaximal bicycle exercise capacity was evaluated in a double-blind randomized study involving 13 patients with chronic heart failure. All patients had impaired maximal exercise capacity (VO2 max = 12.0 +/- 2.6 ml/kg/min) due to fatigue and dyspnea but not angina. The administration of isosorbide dinitrate lowered the resting mean blood pressure (82 +/- 9. mm Hg to 78 +/- 10 mm Hg, (p less than 0.03)) and the resulting pulmonary wedge pressure (26 +/- 5 mm Hg to 12 +/- 6 mm Hg, (p less than 0.01)). Isosorbide dinitrate acutely improved exercise duration during upright bicycle exercise at a workload fixed at 50 percent of the maximal workload (placebo): 21.8 +/- 14.1 min vs isosorbide dinitrate: 31.4 +/- 13.6 min, (p less than 0.003)) due to reduced exertional dyspnea. Administration of chewable isosorbide dinitrate acutely improved submaximal exercise tolerance in patients with chronic heart failure.