The net rate of collagen synthesis in short-term lung explant cultures from rats was examined during a period of 4 wk after administration of a single intratracheal dose of bleomycin. After a delay of 3 to 4 days, the net rate of collagen synthesis was significantly increased over that in untreated control animals. This increase was sustained during approximately 10 days and subsequently returned to normal or below normal values. The increased rate of collagen synthesis could not be explained by increased free radioactive proline pools or by age differences among the animals. The period of increased rate of collagen synthesis preceded the detectable increases in lung collagen content, as we have reported previously.