Due to the lack of information on the effects of inhaled Mt. St. Helens volcanic ash and its potential interaction with sulfur dioxide (SO2), animal studies were performed to determine the acute and chronic health effects of a short-term exposure. This paper describes the inhalation exposure system designed for these studies and theoretically compares the pulmonary deposition in the rats to that in humans. Considering the similarities and differences in regional pulmonary deposition in humans and animals, inhalation studies were performed with fine-mode (<2.5 um aerodynamic diameter, Dae) ash. Comparisons to coarse-mode (>2.5 um Dae) ash were made using intatracheal instillation. A whole-body exposure system was designed to provide inhalation exposures of animals to Mt. St. Helens volcanic ash. SO2, or a combination of both. All exposures were conducted using fine-mode ash samples generated by a Wright dust feed mechanism at a mean concentration of 9.4 mg/m3 (plus or minus) 1.0 SD. Sulfur dioxide was maintained at 2.5 mg/m3 (plus or minus) 0.13 SD. Scanning electron microscopy, X-ray diffraction, and X-ray fluorescence were used to characterize the ash. (Copyright (c) 1985, Academic Press, Inc.)