The benefits of digoxin in patients with atrial fibrillation may be reduced due to its limited effect on atrioventricular conduction. The aim of this work was to compare digoxin and atenolol on functional class, resting and exercise heart rate and exercise capacity in patients with atrial fibrillation. Thirteen subjects with this condition, normal echocardiographic left ventricular function and size, a resting heart rate less than 80 beats/min and with no contraindication for beta blocker or digoxin use were studied. Patients were randomly assigned to receive initially digoxin 0.25 mg o.d. or atenolol 100 mg o.d. in a double blind fashion. The doses were adjusted to obtain a heart rate between 60 and 80 beats/min at the end of the first week of treatment. After two weeks of treatment, outcomes were assessed, patients were left without treatment for one week and crossed over to the other drug after that. Resting heart rates achieved with digoxin and atenolol were similar (67 +/- 11 and 65 +/- 23 beats/min respectively). However, maximal exercise heart rates and maximal exercise time were higher during digoxin treatment (166 +/- 23 vs 135 +/- 27 beats/min and 9.95 +/- 1.68 vs 8.5 +/- 2 min respectively). NYHA functional class deteriorated in three patients receiving atenolol. We conclude that atenolol achieves a better control of heart rate during exercise but also reduces maximal exercise capacity.