A study was undertaken to compare the cardiac and ventilatory responses to different types of exercise between 12 patients with COPD and ten normal age-matched control subjects. Both groups attained comparable heart rates and the percentage of their maximum predicted heart rate. Patients had a higher heart rate and VE with a lower O2P at every level of work load. Patients had a mean VT which approximated their FEV1 and increased their VE predominantly by increasing their respiratory frequency. During the low intensity test, despite the differences in work load, the patients had comparable heart rates and VE. No resting spirometric value accurately predicted work load, VE, or maximal VO2. We conclude that patients have a reduced work tolerance that is not adequately explained by their reduced lung function. Thus, cardiac factors, deconditioning, and the dyspneic sensation may be determinants of exercise limitation in some patients.