If after removal of the cornea, iris, lens and vitreous humor the posterior half of the isolated eye of the frog is kept in the dark for 10 min., practically equal amounts of ammonia (in microgrammes) are found present in both eyes. But when one is kept in the dark and the other is exposed to intense light, the latter shows a marked increase in ammonia content. The ammonia was estimated by distillation in a special all-glass apparatus, and the titration made with very dilute solutions using a mixture of methyl red and methylene blue as indicator. Under certain definite conditions "brei" made from fresh bullock retina forms in a short time considerable amounts of ammonia. In such experiments the addition of adenosin phosphoric acid (the parent-substance of the ammonia of muscular activity) increases the formation of ammonia, while guanosin phosphoric acid has little or no effect, and urea none. Because of the ready susceptibility of adenosin phosphoric acid to the action of fresh surviving retina it might seem that the ammonia-building substance of the retina was also adenosin phosphoric acid, but special experiments to prove this point were negative and the nature of the substance is so far unknown.