Toxicity of the mycotoxin secalonic D (SAD) was examined histopathologically in rats. Dose response was studied by intragastric and intratracheal instillation, and SAD was given in suspension in Krebs‐Ringer phosphate solution at doses well below the reported LD50 values for both rats and mice. A full autopsy was performed on rats sacrificed 1–69 d after instillation. Responses differed markedly depending on the route of administration. Histopathological lesions were observed only in the lungs of rats receiving SAD intratracheally. At 24 h after treatment, the lungs of affected animals showed an acute polymorphonuclear reaction in the distal airways and adjacent alveoli and there was necrosis of airway epithelium. The latter resulted in the development of typical lesions of bronchiolitis obliterans. The initial acute inflammatory response in the peripheral lung became granulomatous at 3 d and the granulomas contained numerous foreign body type giant cells. Between 3 and 7 d the granulomas and bronchiolitis obliterans increased in severity; thereafter they tended to resolve. On termination of the experiment at 69 d there were a few small residual granulomas; however, no significant irreversible pulmonary injury was observed.