During incremental exercise PCO2 does not fall for several work rates ('isocapnic buffering') above the anaerobic threshold (theta an). We explored this apparent lack of compensatory hyperventilation in 24 normal subjects who underwent incremental cycling (15 W/min) to exhaustion. Ventilation, pulmonary gas exchange, and end-tidal gas tensions were computed breath-by-breath. In 10 subjects, arterial blood was sampled every 2 min throughout the test. Our findings confirmed the 'isocapnic' supra-theta an region, but it consistently followed a progressive increase of PETCO2 in the sub-theta an region. A similar pattern was evident for PaCO2. The leveling-out of PETCO2 and PaCO2 was a result of breathing frequency increasing at theta an, thereby shortening expiratory time, i.e., progressively truncating the continued increase in the alveolar PCO2 slope. Consequently 'isocapnic buffering' during incremental exercise does not reflect PCO2 which continues to be regulated at a constant sub-theta an value. Rather it reflects a ventilatory response to the metabolic acidosis which levels a systematically-rising phase of PETCO2 and PaCO2, largely through a change in breathing pattern. Respiratory compensation, as reflected by a declining PETCO2 and PaCO2, does not occur typically for a subsequent 2 or more minutes.