Nine male patients (mean age 65 yr) with chronic atrial fibrillation underwent maximal exercise testing during placebo, β-adrenergic (celiprolol, 600 mg), or calcium (diltiazem, 30 to 60 mg four times daily) channel blockade. The results were analyzed to determine which factors most closely related to ratings of perceived exertion (RPE) during exercise. Heart rate (HR), blood pressure (BP), oxygen uptake (VO2), minute ventilation (V̇E), and carbon dioxide production (V̇CO2) were evaluated at rest, 3.0 mph/0% grade, the gas exchange anaerobic threshold (Atge), 80% of placebo maximal O2 uptake, and maximal exercise. Both β-adrenergic and calcium channel blockade significantly reduced heart rate and systolic blood pressure relative to placebo; these effects were more profound during β-adrenergic blockade and as exercise progressed. Correlation coefficients and estimates of slope were derived for changes in RPE during exercise vs. changes in HR, V̇O2, V̇E, and V̇CO2 during the three treatments (r = 0.76 to 0.92, P < 0.001). Although RPE was significantly correlated with HR during placebo and diltriazem therapy (r = 0.45, P < 0.01), this was not the case during β-adrenergic blockade (r = 0.31, NS). Slope of the regression lines between RPE and ♀EO2, V̇E, and V̇CO2 did not differ between the three treatments. Slope of the regression lines between RPE and HR differed only during calcium channel blockade. Because the presence of atrial fibrillation and β-adrenergic blockade altered the associations between RPE, V̇O2, and HR, these results suggest that V̇E is more closely related to RPE than the other parameters.