AbstractDuring routine inhalation toxicity studies, microscopic examination of tissues from iungs which have shown small, but statistically significant increases, in organ weight has failed to show evidence of any pathological change. Historically, increases, thought to be due to mild oedema, have been difficult to identify microscopically. A method suitable for dealing with large numbers of rodents has been developed, which can detect microscopically changes associated with small increases in lung weight. The lungs were inflated with formalin vapour and fixed in 10 buffered formalin. Histological processing methods were varied to obtain the best demonstration of oedema. Fixation methods were compared using α‐naphthylthiourea‐induced oedematous lungs. The left lobe was inflated with formalin vapour and the rest of the lung was distended with 10 buffered formalin. The best demonstration of oedema followed formalin vapour fixation. Evidence of oedema, indicated by lung weight increases of approximately 10, was seen microscopically in formalin‐vapour‐fixed lungs, but was not seen in the lobes distended with 10 buffered formalin. Application of this technique to other species including cats and dogs has also proved su
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