Morphological evidence has shown that the herbicide paraquat produces severe lung fibrosis in experimental animals. In order to investigate the biochemical basis for this fibrosis, prolyl hydroxylase activity, total hydroxyproline content and collagen and protein synthesis were estimated in rats (200 gm) which were pretreated with paraquat (25 mg/Kg; I.P.) nine days before sacrifice. Prolyl hydroxylase activity in the treated rats was significantly elevated in the lungs. However, no such increase was seen in the kidney. There was no significant change in the total hydroxyproline content or collagen formation in the treated animals. When paraquat was added to the culture medium and incubated with lung slices, there was a marked decrease in collagen synthesis along with a decrease in protein synthesis. When paraquat was added to lung cells in culture, it was found that lung explants were more sensitive to paraquat toxicity than cells obtained by trypsinization. In explants which were sensitive to paraquat, there was a marked decrease in both protein synthesis and collagen synthesis.