Rats were exposed to 5 mg/m3 or 50 mg/m3 (respirable aerosol concentration) volcanic ash, or to 50 mg/m3 quartz, 6 h/day, 5 days/wk, for as long as 12 months. Subgroups of rats were killed at 4-month intervals to determine lesion development as a function of dose and cumulative exposure. Treatment-related lung changes comprised alveolar macrophage accumulation, interstitial reaction, peribronchiolar lymphoid hyperplasia, lymph node hyperplasia, and--in the 50 . mg/m3 ash-exposed and quartz-exposed rats--alveolar proteinosis. Some of the changes did not increase appreciably after 8 months of exposure, whereas others continued to increase as a function of cumulative exposure. Generally, there were distinct dose-response and agent-response relationships. These trends were also observed in lung weights, respiratory rates, and volumes of mediastinal lymph nodes. Like the histologic changes, the effects on these ancillary parameters also increased in the following order: sham-exposed control group, 5 mg/m3 and 50 mg/m3 ash-exposed groups, and quartz-exposed positive control group.