To study the effect of prazosin therapy on left ventricular function in patients with chronic stable heart failure, first pass radionuclide angiography at rest and during exercise was performed in 15 patients before the administration of prazosin and after seven to 12 weeks of prazosin therapy. There was no significant change in resting ejection fraction before and during prazosin therapy (36 +/- 14 per cent versus 37 +/- 14 per cent) (mean +/- standard deviation). However, exercise ejection fraction increased from 34 +/- 14 per cent to 42 +/- 17 per cent (p less than 0.01). The difference in ejection fraction from rest to exercise (ejection fraction response) changed significantly from -2 +/- 6 per cent before prazosin therapy to +5 +/- 7 per cent during prazosin therapy (p less than 0.01). Exercise duration increased from 368 +/- 82 seconds to 476 +/- 82 seconds (p less than 0.01). Total work capacity measured in kilojoules increased from 12.6 +/- 8.3 to 18.6 +/- 10.4 (p less than 0.01). The improved ejection fraction response during prazosin therapy correlated with the improved work capacity (r = 0.69, p less than 0.01) and exercise duration (p = 0.59, p less than 0.05). This improvement occurred despite a significant weight gain with prazosin from 72.2 +/- 20.8 kg to 73.5 +/- 20.8 kg (p less than 0.01). These data suggest that long-term prazosin therapy is effective in the treatment of heart failure. However, the beneficial effects of prazosin, an alpha 1 blocking agent, may be evident only during exercise.