Exercise tolerance inversely correlates with the severity of the disease in patients with idiopathic pulmonary arterial hypertension (IPAH). Cycling and walking protocols are commonly utilized in the evaluation of exercise intolerance in IPAH, but little information exists on possible differences in ventilatory and gas exchange adaptations to these exercise modalities. In a group of patients with moderate to severe IPAH (n = 13), we studied the ventilatory, cardiovascular and gas exchange adaptations to maximal incremental walking (W) and maximal incremental cycling (C). During W, compared to C, the ventilatory equivalents for CO(2) output (V'(E)/V'CO(2)) were significantly higher either expressed as the rate of increment (56 +/- 5 vs. 45 +/- 3; P < 0.0001) or as the absolute values at anaerobic threshold (AT) and at peak exercise. At AT, the increase in V'(E)/V'CO(2) during W was associated with a significant lower value of end-tidal carbon dioxide. At peak W, compared to peak C, dyspnea sensation was higher and arterial oxygen saturation (SpO(2)) was lower (87 +/- 2 vs. 91 +/- 2, P < 0.001). In patients with IPAH the physiologic information obtained with W are different from those obtained with C. Tolerance to W exercise is limited by high ventilatory response and dyspnea sensation. W should be used to assess the degree of lung gas exchange inefficiency and arterial O(2) desaturation during exercise.