OBJECTIVE The aim of this study was to investigate the possible rescue effect of subretinal iris pigment epithelial (IPE) cell transplantation in Royal College of Surgeons (RCS) rats by light and electron microscopic histology. METHODS IPE cells were harvested from 20- to 26-day-old Long-Evans rats and were directly trans planted transsclerally into the subretinal space of 32 16- to 20-day-old RCS rats using a 32-gauge Hamilton syringe. Specimens of transplanted eyes were embedded for electron microscopy after 8 weeks. Specimens from the iris and retinal pigment epithelium (RPE) of Long-Evans rats and RPE from RCS rats without surgical treatment were also embedded. Sham surgery was also performed in 8 eyes. RESULTS The IPE cells transplanted into the subretinal space were localized between host RPE and retina, had round cell shapes without polar organization, and contained phagosomes resulting from rod outer segment (ROS) uptake. The underlying host RPE cells were heavily pigmented. RPE cells from RCS rats revealed fragmentation of endoplasmic reticulum, which distinguishes them ultrastructurally from pigment epithelial cells of Long-Evans rats. Ultrastructural alterations were observed in the cytoplasm of transplanted cells. Melanin granules in the IPE cells were found in large vacuoles, which also contained phagosomes originating from ROS uptake. In 13 eyes, 1 to 4 rows and 5 to 8 rows of saved photoreceptors were detected facing transplanted IPE cells in 6 (46%) and 4 (31%) eyes, respectively, 2 months after surgery. However, in 10 (53%) and 7 (37%) of 19 eyes, 1 to 4 rows and 5 to 8 rows, respectively, were also found at sites without IPE cells in the plane of section. ROS directed toward transplanted IPE cells were seen in one case, but these rods were shortened and disorganized. At most sites between transplanted cells and inner segments of photoreceptors, outer segments and cellular debris were absent. In eyes without transplanted cells no photoreceptor cells were alive at the age of 2 months. After sham surgery 6 (75%) eyes had 1 to 4 rows and 2 (25%) 5 to 8 rows of photoreceptors. CONCLUSIONS Transplanted IPE cells can take up and degrade ROS in vivo in RCS rats. Uptake of ROS alters the morphology of pigment granules in transplanted IPE cells. Pigmentation is an uncertain marker for identifying transplanted pigment cells. IPE transplants are not as good as RPE transplants in rescuing photoreceptors. However, there is a significant difference between transplanted eyes and nontreated eyes. The rescue effect of IPE cells was not significantly different from that of sham surgery.