Six patients with chronic atrial fibrillation (AF) took single doses of digoxin, verapamil and diltiazem, alone and in combination. Three hours after dosing, resting and post-exercise heart rate, exercise tolerance and resting and post-exercise cardiac output were measured. Post-exercise heart rates ranged from 167 bpm (after placebo) to 122 bpm (after digoxin plus diltiazem) (P less than 0.05). However, the lower ventricular rates seen after treatment with the calcium antagonists were not associated with improved exercise tolerance, which did not differ significantly between the various treatments. Reduction of the ventricular rate was associated with a small increase in stroke volume but the benefits of this were offset by a rate related reduction in cardiac output. Further reduction of the rapid ventricular rates seen in digitalized patients with AF does not appear to be of benefit in terms of improving either exercise tolerance or cardiac output.