Hypokalemia as a typical feature of primary aldosteronism (PA) is associated with muscle weakness and could contribute to lower cardiopulmonary fitness. The aim of this study was to describe cardiopulmonary fitness and exercise blood pressure and their determinants during a symptom-limited exercise stress test in patients with PA. We performed a cross-sectional study of patients with confirmed PA who were included before adrenal vein sampling on whom a symptom-limited exercise stress test with expired gas analysis was performed. Patients were switched to the treatment with doxazosin and verapamil at least two weeks before the study. In 27 patients (17 male) the VO(2peak) was 25.4+/-6.0 ml/kg/min which corresponds to 80.8+/-18.9 % of Czech national norm. Linear regression analysis shows that VO(2peak) depends on doxazosin dose (DX) (p=0.001) and kalemia (p=0.02): VO(2peak) = 4.2 - 1.0 * DX + 7.6 * Kalemia. Patients with higher doxazosin doses had a longer history of hypertension and had used more antihypertensives before examination, thus indicating that VO(2peak) also depends on the severity of hypertension. In patients with PA, lower cardiopulmonary fitness depends inversely on the severity of hypertension and on lower plasma potassium level.