Rats were exposed by inhalation to 9.4 mg/m3 size-fractionated volcanic ash for 5 days (2 hr/day) and examined for changes in pulmonary function and histology for periods of up to 1 year. Fine-mode volcanic ash, SO2, and a combination of ash and SO2 produced no observable effects in normal rats and rats with elastase-induced emphysema. However, there was a mild irritant response to SO2 which was not influenced by the volcanic ash. Rats injected intratracheally with fine-mode volcanic ash or saline showed no evidence of pulmonary alterations after 6 months. Those injected with coarse-mode volcanic ash showed minor pulmonary functional changes, histologically detectable alveolitis, and small increases in lung weight. In contrast, quartz-injected rats showed large alterations in pulmonary function, lung weight, hydroxyproline levels, and large areas of lung consolidation and fibrosis.