In rat lungs, 24 h after a 10 min inhalation of a nebulized 1% (w/v) trypsin solution, there was a 25% increase in lung weight. The incorporation of 3H-tryptophane and 2,3-[3H]-proline into trichloroacetic acid insoluble material was decreased although there was no alteration in prolyl hydroxylase activity. Although hydroxyproline formation was decreased, this decrease was probably due to the general decrease in protein synthesis. Ninety-six hours after inhalation of the trypsin solution there was an increase in non-collagen protein biosynthesis. Proline incorporation and hydroxyproline formation were both increased more than the tryptophane incorporation increase at this same time point. These increases were accompanied by an increase in prolyl hydroxylase activity. These experiments indicate that major changes in protein biosynthesis occur in lung tissues after inhalation of proteolytic enzymes and demonstrate the temporal biochemical changes which occur in lung injury.