In order to see if exposure to cold temperature affects the development of bleomycin lung toxicity in mammals, 14 rats received endotracheal injections of bleomycin, while another 14 rats received saline. Half of the saline-treated animals were then exposed to room temperature for 7 days (SRT), and half were exposed to 4 degrees C for 7 days (SC). The bleomycin-treated animals similarly yielded groups kept at room temperature (BRT) and in the cold (BC). After 1 wk the rats were killed and the lungs were sectioned for assessment of histologic changes. All the bleomycin-treated animals had evidence of lung toxicity but this was much less marked in group BC than in group BRT. The lungs in group SC remained normal. These results were confirmed by morphometric methods, which showed significantly increased volume proportions of alveolar walls in groups BRT and BC compared with that in groups SRT and SC (p less than 0.001), with BRT being greater than BC (p less than 0.001). It is concluded that exposure of bleomycin-treated rats to low ambient temperature significantly reduces lung toxicity.