We have analyzed the magnitude and uniformity of the expression of manganese superoxide dismutase (Mn SOD) protein in alveolar type II cells of rats exposed to hyperoxia using quantitative colloidal gold immunocytochemistry and morphometric techniques. Sprague-Dawley rats were exposed continuously to normal air or to 85% oxygen for 7 and 14 days. The lungs were fixed by intratracheal instillation of a paraformaldehyde-glutaraldehyde fixative. Lung samples were dehydrated in ethanol and embedded in LR-White. Thin sections for electron microscopy were labeled with anti-rat Mn SOD rabbit antiserum followed by protein-A gold. The labeling density (gold particles/micron 2) over subcellular compartments was determined, and relative organelle volumes were measured using a random point overlay. The results confirm that alveolar type II cells are a locus of Mn SOD response in the lungs of hyperoxic rats and contribute to this response through a combination of changes: an increase of Mn SOD concentration in the mitochondrial matrix, an increase of mitochondrial volume per cell, and type II cell hyperplasia.