In the Royal College of Surgeons rat with inherited retinal dystrophy, vascularization of the retinal pigment epithelium (RPE) is preceded by migration and proliferation of Müller cell processes in the subretinal space where they contact the RPE. Later, RPE cells envelope subretinal vessels which have lost their perivascular Müller cell sheath. To characterize RPE cell changes and interactions in relation to glial and vascular transformations in retinal dystrophy, we used immunocytochemical techniques and antibodies against cytokeratin (CK) and glial fibrillary acidic protein (GFAP). Prior to the proliferation of Müller cell processes in the dystrophic retina, CK filaments in RPE cells formed a circumferential meshwork with intense cytoplasmic and perinuclear labeling as in control RPE cells. Following entry of Müller cell processes into the membranous debris zone and formation of RPE-Müller cell contact, RPE cells became pleomorphic and extended prominent apical processes in the debris zone. Some CK-reactive RPE cells detached from Bruch's membrane and migrated into the debris zone. Electron microscopic study showed extensive areas of close RPE-Müller cell contact at this time. Obvious junctional specializations of the plasma membranes were not seen but prominent tubulo-vesicular profiles occupied the cytoplasm of altered RPE and Müller cell processes. Following RPE vascularization, hypertrophic CK-positive cells surrounded blood vessels and accompanied them into the inner retina. Electron microscopic analysis showed that RPE-associated vessels were fenestrated and devoid of their perivascular glial sheath. Apparent proliferation of RPE cells and redistribution of CK filaments were observed. Our study shows that RPE cell alterations accompany Müller cell and vascular changes which result in altered RPE-Müller cell and RPE-endothelial cell relationships in the dystrophic rat retina. The altered relationships among RPE, Müller and endothelial cells may result in increased cellular interaction and promote proliferation and transformation of all three cells types in diseased retinas.