Male rats were exposed to 0.125, 0.25 or 0.5 ppm of ozone or clean air for up to 1 year. During this exposure period there was little evidence for collagen accumulation in the lungs. However, the rate of incorporation of tritiated proline into both lung collagen and total lung protein was accelerated. These data suggest that exposure to ozone under these conditions results in an increase in the turnover of collagen as well as other lung proteins.