Twelve dogs were anaesthetized with pentobarbitone and exposed to predetermined concentrations of nitric oxide and nitrogen dioxide. Three dogs recovered and were sacrificed, one at 24 hours and two at 48 hours. Autopsy was performed and the morbid anatomy of the lungs is described in detail. The changes were focal, including oedema, hyperinflation, haemorrhage, desquamation of mucosa and bronchopneumonia. The severity of the lesions was broadly proportional to the duration and concentration of gaseous exposure and the most severe effects were seen in two of the survival dogs.