End-expiratory pressure is often used to improve arterial oxygenation and prevent atelectasis in intubated spontaneously breathing patients. To compare the effect of extubation from low levels of expiratory positive airway pressure (EPAP) of extubation from ambient airway pressure, functional residual capacity (FRC) and arterial blood oxygen tension (Pao2) were measured in 12 spontaneously breathing patients during three conditions in the peri-extubation period: 1) intubated at 5 cm H2O EPAP (EPAP 5); 2) intubated at ambient airway pressure (EPAP 0); and 3) within one hour after extubation. During EPAP 5, mean +/- SE values for FRC (1864 +/- 230 ml) and Pao2 (114 +/- 8 torr) were the same as those obtained after extubation (FRC = 1794 +/- 159 ml, Pao2 = 117 +/- 5 torr). However, both FRC (1600 +/- 186 ml) and Pao2 (106 +/- 8 torr) were lower during EPAP 0 than after extubation or EPAP 5 (P less than 0.01-0.05). The magnitude of increase in FRC and Pao2 on extubation from EPAP 0 varied inversely with the patient's lung thorax compliance (r = -0.84, P less than 0.005). It was concluded that a period of EPAP 0 is not necessary in the weaning period, and that it may be deleterious in patients with compromised lung thorax mechanics.