All cats fed a taurine-free casein diet for 23 weeks have shown a nondetectable electroretinogram (ERG) in association with a plasma and retinal taurine deficiency. In the present study, the casein diet was supplemented with either taurine or taurine precursors (methionine or cysteine) for 23 weeks to see if retinal function would be preserved. Cats fed the casein diet supplemented with methionine or cysteine showed ERG's reduced in amplitude and delayed in implicit time and had plasma and retinal taurine levels that were well below normal by 23 weeks. Only those cats given taurine in the diet (i.e., those fed chow or casein supplemented with taurine) retained normal ERG function and normal plasma and retinal taurine concentrations. These findings establish a role for taurine in maintaining normal retinal function in the cat.