Image analysis quantitation of cytochrome oxidase activity, cytochemicaUy visualized in mitochondria, and the comparison between the activities of pulmonary alveolar Type I and Type II cells were performed. Normal lungs of rats were fixed with 2% glutaraldehyde for 10 min at 4°C and then small blocks were incubated for 60 min at 37°C in a medium containing 1.0 mg/ml (2.8×l0⁻⁴M) 3,3′-diamiDobcnzidlnc-tctrahydrochloride, 1 mg/ml cytochrome c, 0.1 mg/ml catalase, 7% sucrose and 0.1 M phosphate buffer at pH 7.4. The intermembrane space, between the outer and inner membranes, and intracristal space of Type II cell mitochondria were filled with deposits whose mean volume was 0.047 μm³. Type I cell mitochondria were smaller in size than those of Type II cells, but because reaction deposits were not fully accumulated within the spaces the resulting net deposit volume was 0.00182 μm³ If the intermembrane spaces of Type I cell mitochondria had been thoroughly filled by the deposits, the mean volume would have been 0.00248 μm³. These results indicate that stereoscopic image analysis of the reaction product is appropriate in the quantitation of cytochemicaUy visualized enzyme activity, and furthermore suggest that Type I cells may be rather deficient in cytochrome oxidase activity.