The isolation of the retinal vessels by the trypsin digestion technique1 allows histochemical studies on this structure. Recently, histochemical studies of several enzyme groups, including the dehydrogenase enzymes, have been reported.2 The present study is a more detailed study of these dehydrogenase enzymes in the rat, cat, and dog with nitro-blue tetrazolium (Nitro-BT). Particular attention has been given to the problem of tetrazolium penetration. Materials and Method The eyes of albino rats, cats, and dogs were studied. Under ether or pentobarbital (Nembutal) anesthesia, the eyes were quickly enucleated, rinsed, and frozen at —75 C in an isopentane, acetone, solidified CO2 (Dry Ice) system. Thawing was done in air at room temperature. Some eyes were repeatedly frozen and thawed, while others were used in the fresh state. The globes were stored in a freezer at —25 C and were used within six days. For incubation, the eyes were