The study investigates the renewal of photoreceptor cell outer segments in rats, mice, and frogs using radioautography to track the movement of labeled methionine-3H. Initially, the labeled amino acid accumulates in the inner segment of the photoreceptor cells. Within 24 hours, it moves to the base of the outer segment, forming a distinct reaction band that gradually moves towards the cell's apex, where it disappears. This process indicates that the outer segment is continuously renewed through the repeated addition of new material (membranous discs) at the base and the removal of material at the apex. The rate of outer segment renewal is influenced by temperature and illumination intensity. In frogs, the rate increases with higher ambient temperatures, and in both frogs and rats, it is elevated under conditions of increased retinal illumination. The findings suggest that the renewal mechanism involves the continuous formation of new discs at the base of the outer segment and their gradual displacement towards the apex, with the material at the apex being removed by the retinal pigment epithelium.The study investigates the renewal of photoreceptor cell outer segments in rats, mice, and frogs using radioautography to track the movement of labeled methionine-3H. Initially, the labeled amino acid accumulates in the inner segment of the photoreceptor cells. Within 24 hours, it moves to the base of the outer segment, forming a distinct reaction band that gradually moves towards the cell's apex, where it disappears. This process indicates that the outer segment is continuously renewed through the repeated addition of new material (membranous discs) at the base and the removal of material at the apex. The rate of outer segment renewal is influenced by temperature and illumination intensity. In frogs, the rate increases with higher ambient temperatures, and in both frogs and rats, it is elevated under conditions of increased retinal illumination. The findings suggest that the renewal mechanism involves the continuous formation of new discs at the base of the outer segment and their gradual displacement towards the apex, with the material at the apex being removed by the retinal pigment epithelium.