All cats fed a taurine-free casein diet for at least 23 weeks have shown granularity with a hyper-reflective white zone in the area centralis, nondetectable electroretinograms (ERG's), and structural changes indicating photoreceptor cell degeneration. The present study has demonstrated that cats fed this casein diet have a selective decrease in plasma and retinal taurine concentrations by five weeks; taurine levels were about 4 per cent of normal in plasma, and 60 per cent of normal in retina. After 10 weeks, taurine levels were 2 to 4 per cent of normal in plasma and reached a minimum of 20 to 30 per cent of normal in the retina. These biochemical changes occurred in association with a delay in the cone ERG implicit time at five weeks and reduced cone and rod ERG amplitudes at 10 weeks. During this period, retinal DNA content (as a measure of cell viability) and fundus appearance were normal. By 23 weeks, ERG's were nondetectable, retinal DNA content was reduced, and the fundus showed typical changes in the area centralis. These studies help to establish a biological role for taurine in maintaining photoreceptor cell function and viability in the cat.