Respiratory gas exchange on exercise was evaluated as a non-invasive method of assessing patients with heart failure. Twenty four men (age 28-72) with symptomatic chronic stable heart failure (New York Heart Association class II-III) and ten controls aged 36-70 were studied. During treadmill exercise oxygen consumption and carbon dioxide production were measured continuously by analysis of mixed expired gas with a computerised mass spectrometer. The anaerobic threshold was defined as the oxygen consumption at which carbon dioxide production increased disproportionately in relation to oxygen consumption. Oxygen consumption was stable at rest and increased on exercise, reaching a steady state within three minutes of any change in workload. The measurements of maximum oxygen consumption at the end of exercise and of anaerobic threshold were reproducible (retest reliability coefficients 90% and 91% respectively). There were significant differences in maximum oxygen consumption between functional classes. Similarly, there were significant differences in anaerobic threshold between classes, though there was considerable overlap. Measurement of oxygen consumption and anaerobic threshold provides an objective noninvasive assessment of patients with heart failure.