The ventilatory response to exercise includes changes in tidal volume as well as in breathing frequency. These changes together underlie the highest rate of pulmonary ventilation to match the metabolism requirement during exercise. Normally exercise ceases when ventilation is still increasing despite oxygen uptake is steady at its maximal level. The increased oxygen uptake and carbon dioxide production at the last periods of severe exercise coincide with lower and higher values of carbon dioxide and oxygen percentages respectively in the expired air. Simultaneously, the breathing frequency steeply augments while the oxygen partial pressure in the arterial blood fall and the alveolar oxygen concentration increase. To check the homeostatic role of high rate of ventilation at the end of vigorous exercise we tested the pulmonary gas exchange and blood gases of 8 subjects during severe exercise and under unloaded conditions simulating those obtained during severe exercise for a short period. Results show that high breathing frequency in unloaded conditions reduced the carbon dioxide fraction (from 4.5 to 2.5%) and increased the oxygen fraction (from 2.7 to 3.85%) in the expired air in a fashion compatible with those observed during severe exercise. On the other hand, low values of arterial oxygen partial pressure (79_ 3.5 mmHg), venous pH (7.05_ 1.5) and plasma bicarbonate concentration (14_ 3.2 mEq_l-1) during strenuous exercise are pointing out the limit of buffering capacity of the subjects. The ventilation at the end of highest intensity of exercise mainly consists on an increased respiratory frequency with a steady tidal volume (ca. 70% of vital capacity) that would lead to an inefficient ventilation from the view of blood-alveolar gas exchange but protective as respect the buffer reserve in the plasma compartment. Nevertheless, the effects of the high proton load produced by muscle contractions probably continued acting upon the central chemosensitive neurons as reflected by the increase in the breathing frequency during exhausting exercise. The above joined to the high cost of ventilation at high frequency with mild tidal volume will lead to a positive feedback that exhaust the subject before the homeostasis is fatally impaired.