Mice were injected intratracheally with silica or Mt. St. Helens volcanic ash (0.2 mg/g body weight) and examined 6 months later for changes in lung function, histology, and hydroxyproline content. Results were compared with a similar study using rats. Mice injected with volcanic ash showed significant changes only in wet lung weights. Those injected with silica showed an approximate doubling of lung wet weight and dry weight and hydroxyproline content. Larger increases were seen if lymph nodes were left attached. Lung compliance, total lung capacity and the shape of the pressure-volume curve of the lung were changed as much as 22% in the silica treated mice. A mild degree of fibrosis with no dense lung consolidation was noted microscopically in silica treated mice. In contrast, silica treated rats showed dense lung consolidation, 3-5 fold increases in lung weights and hydroxyproline content, and up to 40% reductions in pulmonary function measurements.