Large differences exist between maximum theoretical values for the O2debt following muscular exercise (3–5 liters) and some experimentally determined values (12–20 liters). This study was designed to observe the ventilatory responses during recovery from muscular work and their relation to O2debt. Two subjects were tested over a wide range of work loads on a bicycle ergometer. O2uptake was determined by a modification of the Douglas Bag technique. In a series of experiments on one subject, O2debt increased approximately as a linear function of work load through about 1200 kgm/-min, at which point the O2debt began to increase disproportionately. The largest O2debt was 8.9 liters. The rate of ventilation was extremely high during severe work and remained elevated during the first few minutes of recovery, suggesting that the O2cost of breathing during recovery might influence the measurement of O2debt. We conclude that there exist during recovery, factors unrelated to the replenishment of depleted energy stores (such as the O2cost of breathing), which may influence the estimation and interpretation of O2debt. Furthermore, until this problem can be resolved, the use of O2debt measurements as an index of anaerobic energy stores available during severe work is suspect.