Certain aspects of the metabolism of lung proteins during experimental berylliosis were studied. The increase in the inclusion of tagged amino acids in the insoluble lung proteins of test rats apparently is attributable to the increase in the synthesis of these proteins. The significant increase in the dry weight of the lungs, coinciding in time with the high inclusion of lysine in the insoluble proteins, is an indication that favors this fact. The conclusion derived from the description of the pathological picture, in which proliferation of the cellular elements and the appearance of collagenous fibers may be noted from the first day of development of experimental berylliosis, is that the protein development process in the lungs is an intensive one. Lysine and glycine, used in the experiment, along with proline and oxyproline, are the basic amino acids from which collagen forms. Increase in the inclusion of tagged amino acids can take place when there is a change in the conformation properties of the protein as a result of a disruption in protein synthesis, or as a result of different denaturation and prenaturation changes in the proteins.