In patients affected by Creutzfeldt-Jakob disease and in animals affected by transmissible spongiform encephalopathies, retinal functions are altered, and major spongiform changes are observed in the outer plexiform layer where photoreceptors have their synaptic terminals. In the present study, the prion protein PrP c was found to form aggregates in rod photoreceptor terminals from both rat and human retina, whereas no labeling was observed in cone photoreceptors. Discrete staining was also detected in the inner plexiform layer where the prion protein was located at human amacrine cell synapses. In mixed porcine retinal cell cultures, the PrP106-126 prion peptide triggered a 61% rod photoreceptor cell loss by apoptosis as indicated by terminal deoxynucleotidyl transferase dUTP nick-end labeling, whereas cone photoreceptors were not affected. Amacrine cells were also reduced by 47% in contrast to ganglion cells. Although this cell loss was associated with a 5.5-fold increase in microglial cells, the strict correlation between the PrP c prion protein expression and the peptide toxicity suggested that this toxicity did not rely on the release of a toxic compound by glial cells. These results provide new insights Transmissible spongiform encephalopathies in humans and in animals are linked to the transconformation of the prion protein PrP c into a proteinase-resistant form PrP res . 1 These neurological disorders exhibit common pathological symptoms like vacuolization of the neurophils, astrocytosis, and loss of neurons. 2 In patients with CreutzfeldtJakob disease, alteration of the retinal function was attested by the decrease in the electroretinogram b-wave amplitude, 3-5 which reflects the activity of bipolar cells postsynaptic to photoreceptors. These early electroretinogram changes were even proposed as a diagnostic measurement for the disease. 6 In histology, major spongiform changes were observed in the outer plexiform layer (OPL) where photoreceptors have their synaptic terminals, and only moderate changes were observed in the inner plexiform layer (IPL) and ganglion cell layer. 5 These localizations were consistent with the reported accumulation of the pathogenic form of the prion protein, PrP res , throughout the plexiform layers of the human retina. 7 The in vivo retina has been used on many occasions to study the progression of the disease in animal models because of its organized structure, its in vivo access for intraocular injection, and the possibility to correlate the