Retinal ganglion cells in the rat were studied using the heavy metal intensified cytochrome oxidase and horseradish peroxidase histochemical methods. The results show that a population of large retinal ganglion cells was consistently observed with the cytochrome oxidase staining method in retinas of normal rats or rats which received unilateral thalamotomy at birth. These cytochrome oxidase rich ganglion cells appeared to have large somata, 3--6 primary dendrites and extensive dendritic arbors, and are comparable to ganglion cells labeled by the wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). However, the morphological details of some of the cells revealed by the cytochrome oxidase staining method are frequently better than those shown by the HRP histochemical method. These results suggest that the mitochondrial enzyme cytochrome oxidase can be used as a simple but reliable marker for identifying and studying a population of retinal ganglion cells with high metabolic rate in the rat. Retinal ganglion cells in the rat were studied using the heavy metal intensified cytochrome oxidase and horseradish peroxidase histochemical methods. The results show that large population of large retinal ganglion cells was consistently observed with the cytochrome oxidase staining method in retinas of normal rats or rats which received unilateral thalamotomy at birth. These cytochrome oxidase rich ganglion cells have to large somata, 3-6 primary dendrites and extensive dendritic arbors, and are comparable to ganglion cells labeled by the wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). These results suggest that the mitochondrial enzyme cytochrome oxidase can be used as a simple but reliable marker for identifying and studying a population of retinal ganglion cells with high metabo lic rate in the rat.