The effects of normobaric oxygen exposure were investigated in budgerigars (Melopsittacus undulatus). Sixty birds were randomly divided into four equal groups of 15. These groups were randomly allocated as control, acute exposure, repeated acute exposure or chronic exposure. Control birds were exposed to 72 continuous hours of 21% oxygen in a sealed, enclosed chamber. Acute exposure, repeated acute exposure and chronic exposure groups were exposed oxygen at minimum concentration of 95% for a single 3-h period, a 3-h period daily for three sequential days, or a single 72-h period in a sealed, enclosed chamber, respectively. Oxygen exposure resulted in significant alteration in the histological morphology of respiratory exchange tissue, with severe oedema, and inflammatory cell infiltration. Electron micrographs revealed thickening of the blood-gas barrier with the tissue harmonic thickness increasing from 226 ± 90 nm in control birds to 639 ± 393 nm following repeated acute exposure, with the total harmonic thickness increasing from a control value of 345 ± 146 nm to 837 ± 423 nm at the same time. Chronic oxygen exposure resulted in significant changes in cell morphology including thickening of endothelial cells, ruffling of type I respiratory endothelial cells and interstitial vacuolation. These results indicate that budgerigars undergo significant morphological and ultra-structural changes in respiratory exchange tissue following exposure to 100% oxygen.