Histological special staining and dehydrogenase histochemical techniques were used to observe the structure and staining reactions at the junctions of side-arm branches of retinal arteries in man and several species of animals. Annular stains occurred at the junctions of side-arm branches in cat, rat, pig and dog, but not in man, rhesus monkey and beef with PAS, Masson's and elastic Van Gieson's (EVG) stains. However, in man and all the animals studied, they can be demonstrated with dehydrogenase histochemical techniques, i.e. succinate, malic and lactic dehydrogenases (SDH, MDH and LDH). The tissue components of retinal arteriolar annulus and its role in control of retinal circulation were discussed.