Twelve patients (including 2 females) with chronic atrial fibrillation were entered into a randomized, double blind crossover study to compare the effects of digoxin and verapamil upon heart rate, exercise tolerance and symptom control. The dose of digoxin was adjusted so as to give serum concentrations within the range 1.3 to 2.6 nmol l-1 between four and six hours after dosing, and was continued for six weeks. The dose of verapamil was increased from 40 mg tds to 80 mg tds to 120 mg tds at fortnightly intervals. Three patients did not complete the study; two because of adverse effects attributable to verapamil. In the remaining nine patients, mean post exercise heart rates were significantly lower during treatment with verapamil 80 mgs tds (126.7 bpm) than with verapamil 40 mg tds (148.6 bpm) or digoxin (146.7 bpm). However, exercise tolerance was similar with both verapamil and digoxin and the superior control of exercise induced tachycardia achieved with higher doses of verapamil was not associated with improved exercise endurance. Visual analogue scale scores for constipation were significantly higher during treatment with verapamil; scores for other possible side effects and for overall wellbeing were similar. The results do not confirm the findings of others who have reported that verapamil is superior to digoxin in the treatment of atrial fibrillation.