In order to see whether systemically administered N-acetyl cysteine (NAC) could protect the lung from bleomycin lung injury, 24 rats were given a constant subcutaneous infusion of NAC (mean 195 mg.kg-1.day-1) for 7 days while 23 rats were given a similar volume of saline. Two days after the start of infusion, half of each group received an endotracheal injection of bleomycin and the other half received a similar volume of saline. All animals were killed 7 days after intratracheal injection and their lungs were prepared for wet-weight to dry-weight ratio (W:D) and morphometric assessment of histological changes. In animals given intratracheal saline, NAC infusion had no effect on weight gain, W:D or morphometry compared to those animals given saline infusions. However, all bleomycin-treated animals had obvious evidence of lung damage. Compared to either group of animals treated with intratracheal saline, the bleomycin-treated groups gained less weight (p less than 0.001), had a higher W:D (p less than 0.001) and an increase in the volume densities of alveolar and duct walls (p less than 0.001), intra-alveolar cells (p less than 0.05) and consolidation (p less than 0.001). There were no significant differences between the two bleomycin-treated groups in any parameter measured. It is concluded that administration of NAC by a constant subcutaneous infusion was not protective against bleomycin lung injury.