Oral verapamil has previously been shown to reduce heart rate at rest and during mild exercise in chronic atrial fibrillation. Its efficacy in improving cardiovascular performance at higher levels of exercise and its safety were investigated in a prospective, randomized, placebo controlled double-blind study preceded by an open label titration phase in 20 digitalized patients with chronic atrial fibrillation. Maximal exercise capacity was improved (from 522 ± 257 to 806 ± 348 work units, p < 0.0005) when tested by a standardized multistage ergometry exercise test. Heart rate was also reduced at rest, at the end of 3 minutes of 300 KPM exercise, and at the point of maximal exercise. Blood pressure and double product were also reduced. Its efficacy and safety may make verapamil the treatment of choice in chronic atrial fibrillation.