It has been suggested that the oxygen consumption of the respiratory muscles (VO2 resp) may play a role in limiting exercise performance in both healthy subjects and those with chronic airflow limitation (CAL). In order to measure VO2 resp reproducibly at both rest and on exercise, ventilation (VE) and total oxygen consumption (VO2) in 3 normal subjects and in 3 patients with cystic fibrosis were measured while breathing air and again when VE was stimulated by the addition of CO2 to the inspired gas. Since external work was the same it was assumed that any changes in VO2 would be due to the increase in VO2 resp during stimulated breathing allowing for the calculation of VO2 resp. The oxygen cost of breathing was higher in the patients with the increasing ventilation of exercise. These values were reproducible on repetitive measurements. It is concluded that the method employed is applicable in normal subjects and in patients with CAL, and that the O2 cost of breathing is higher in patients with CAL. The O2 cost of breathing increases as VE increases but even during exercise the VO2 resp is only a small fraction of the total VO2 and is unlikely to be a determining factor limiting exercise performance in either normal subjects or those with CAL.