Over a 10-year period, we have studied the Tibetan terrier's visual electrophysiology, light and electron microscopic (EM) retinal characteristics of a slowly evolving inherited ceroid lipofuscinosis (CL). The retinal degeneration with CL inclusions (rdi) in the inner nuclear layer (bipolar cells) and nerve fiber layer (ganglion cells) has been called "rdi" to differentiate the visual abnormality from typical early retinal degeneration (erd) reported also in the Tibetan terrier. The unique "rdi" electroretinogram (ERG) gives a predominant P III wave at age 7 weeks but becomes more characteristically depressed in all phases over several years. Nyctalopia is the only functional abnormality for the first 5 to 6 year of life. Signs are remarkably few considering the pathology. Microscopic studies of the retina show accumulations, increasing with age, of autofluorescent dense inclusion particles which stain intensely by Luxol fast blue, PAS, and acid-fast procedures. Ultrastructural studies of the retina show the dense particles to be lamellar membranes repeating every 2 to 4 nm, consistent with ceroid lipofuscin. The inner retinal layers were always the target layer to be affected first and most severely. The ganglion cells were most frequently involved. The photoreceptors eventually degenerated but relatively few particles were found in this layer. The cytosomes in the cerebral cortex and brainstem neurons resemble lipofuscin, containing granular, lamellar, and globular components. Different pigment bodies were present in the cerebellar Purkinje cells. Neuronal loss which was moderate in the cerebellum and mild in the cerebrum was accompanied by astrogliosis and a striking presence of macrophages.(ABSTRACT TRUNCATED AT 250 WORDS)