A mouth-borne heat and moisture exchanger (HME) was tested. Nine healthy subjects performed an incremental-load cycle ergometry test to exhaustion, breathing once through the HME and once through a similar device without heat-exchange function (control). HME substantially increased inspired gas temperatures and decreased expired gas temperatures measured at the mouth; at 260 W (pulmonary ventilation (VE) approximately 1001 min-1) these changes amounted to + 15 degrees C and -5 degrees C, respectively. The breathing resistance was increased by HME but remained well within tolerable levels even during severe exercise. This was reflected in the subjective assessments of breathing resistance and breathing discomfort which, at any given workload, were rated similarly in the HME and control conditions. Also, time to exhaustion as well as oxygen uptake and VE at a given workload were unaffected by HME. That even at high pulmonary ventilations HME provided a good heat-exchange function while keeping breathing resistance relatively low suggests HME to be a useful aid for individuals suffering from cold-induced bronchospasm.