The fibrogenic potential of vermiculite (1318009) was evaluated in male rats injected intratracheally with 0 or 50 milligrams vermiculite or quartz (14808607) dust. Quartz was administered as a positive control. Selected rats were killed 1, 3, 6, 9, or 12 months later and the lungs and hiliar lymph nodes were removed and examined for histopathological changes. The collagen content of the lungs was determined. The lymph nodes of quartz treated animals were enlarged, with the degree of enlargement increasing progressively with time after exposure. Inflammatory cells were noted in the lymph nodes 1 month after quartz exposure. Typical fibrotic lesions were observed after 12 months. In vermiculite treated rats, slight enlargement of the hiliar lymph nodes were noted after 1 month. Emphysematous changes were noted in the lung parenchyma after 9 months. A few collagenous fibers were observed in the lung and lymph tissue after 3 and 6 months. Significant numbers of reticular fibers were also observed. Collagenous fibers were also noted in the alveoli, bronchi, and smooth pulmonary blood vessels after 12 months. The collagen content of the lungs of vermiculite treated rats increased gradually with time after exposure. The collagen content of the lungs of quartz treated animals increased to a much greater extent 9 and 12 months after exposure. The authors conclude that vermiculite dust causes slight fibrogenic changes in rats. The changes are less severe and progress at a slower rate than those due to quartz.