Constant work rate exercise testing has been used to assess effectiveness of therapeutic interventions in chronic obstructive pulmonary disease. It has been noted to yield larger fractional increases in exercise tolerance than other measures. Reasons for this are rooted in physiological determinants of the time course of pulmonary ventilation. Following exercise onset, ventilation increases in three phases; the slowest phase is seen only at high exercise intensities. In chronic obstructive pulmonary disease exercise proceeds until ventilation reaches a limiting value. Because both ventilatory requirement for exercise and limiting ventilation can be manipulated by several interventions (exercise training, oxygen inhalation, bronchodilator administration), constant work rate tolerance responds to these interventions. The power-duration relationship predicts the effect of the work rate imposed on tolerated duration of a constant work rate test. Arguments are presented that a pre-intervention constant work rate duration of 4-7 minutes is desirable. At present, the recommended strategy to achieve this target duration is to choose a work rate equal to 85% of peak work rate achieved in a constant work rate test. There is clearly insufficient information to reliably determine a minimal clinically important difference for this test. A lower bound of 1.75 minutes is suggested.