The effects of two calcium channel blockers, diltiazem (270 mg/day) and verapamil (240 mg/day), were studied in 18 patients with chronic atrial fibrillation. During 24 h Holter electrocardiographic monitoring, mean ventricular rate (beats/min) decreased from 88 +/- 14 with placebo to 76 +/- 13 (p less than 0.001) with diltiazem and 80 +/- 11 (p less than 0.01) with verapamil. Maximal symptom-limited exercise tolerance (W) increased from 127 +/- 39 during the placebo period to 136 +/- 42 (p less than 0.01) with diltiazem and 137 +/- 39 (p less than 0.01) with verapamil. Ventricular rate and rate-pressure product were lower at rest and during exercise with diltiazem and verapamil than with placebo (p less than 0.001), with the drugs being similarly effective. Ventricular rate at maximal exercise (beats/min) was 179 +/- 13 with placebo compared with 159 +/- 21 with diltiazem and 158 +/- 23 with verapamil. Maximal oxygen uptake (ml/kg per min) was 22.3 +/- 4.5 with placebo, 23.7 +/- 4.9 (p less than 0.05) with diltiazem and 22.9 +/- 4.5 with verapamil (p = NS). Respiratory gas exchange anaerobic threshold was reached at a work load (W) of 76 +/- 21 with placebo, 84 +/- 27 (p less than 0.05) with diltiazem and 85 +/- 23 (p less than 0.01) with verapamil. In conclusion, patients with chronic atrial fibrillation have modestly improved exercise tolerance with calcium channel blockade therapy. The dromotropic responses and the effects on physical performance are of similar magnitude for diltiazem and verapamil.