During progressive exercise ventilation (VI) initially increases through increases in both tidal volume (VT) and respiratory frequency (f) but at high levels of exercise further increases in VI are almost completely due to increases in f and a VT plateau is seen. We wished to determine whether the presence of the VT plateau is due to a tachypneic influence related to very high levels of exercise or whether it represents a stereotypic response of the respiratory system at high levels of VI. We therefore compared breathing pattern in six subjects during maximal incremental exercise (ME) with that in the same subjects when similar levels of VI were obtained by a combination of submaximal exercise and hypercapnia (E/CO2). A VT plateau was seen in all ME and E/CO2 tests. There was no significant difference in the level of the VT plateau between the ME (2.93 +/- 0.17 liters) and E/CO2 (2.97 +/- 0.12 liters) tests. We conclude that the presence and level of the VT plateau during ME is not due to a tachypneic stimulus related to very high levels of exercise but is a function of the level of VI.