The 6-min walking test (6MWT) is frequently used to assess functional capacity in chronic cardiopulmonary disorders because of its simplicity. The study examines the physiological responses during encouraged 6MWT in patients with chronic obstructive pulmonary disease. Pulmonary oxygen (O2) uptake (V'O2) was measured in 20 male patients (age 66+/-6 yrs, forced expiratory volume in one second 45+/-14% predicted) during 6MWT and incremental cycling, in random order. O2 tension in arterial blood, carbon dioxide tension in arterial blood and arterial lactate concentration ([La]art) were obtained in the last 10 patients. During the 6MWT, V'O2 showed a plateau after the 3rd min (1.39+/-0.28, 1.42+/-0.31, and 1.40+/-0.30 L x min(-1), 4th, 5th and 6th min, respectively), and minute ventilation (V'E) (42+/-8 L x min(-1)) was 91% maximal voluntary ventilation. No differences were shown between 6MWT (6th min) and peak cycling exercise in V'O2 (1.40+/-0.30 versus 1.41+/-0.28 L x min(-1), respectively), cardiac frequency (126+/-13 versus 130+/-12 beats x min(-1)), or arterial respiratory blood gases. The two tests were significantly different in V'E (42+/-8 versus 47+/-8 L x min(-1), 6MWT versus cycling, respectively), carbon dioxide production (1.30+/-0.31 versus 1.45+/-0.18 L x min(-1)) and [La]art (2.9+/-1.99 versus 5.9+/-1.51 M). The study demonstrates that an encouraged 6-min walking test generates a high but sustainable oxygen uptake. Since the oxygen uptake plateau reflects the integrated response of the system, it may explain the high prognostic value of the 6-min walking test.