Cats fed a purified diet containing purified casein as the source of protein develop retinal degeneration due to the lack of taurine in the diet. To test whether cats can synthesize this sulfur amino acid from sulfate or cystine, radioisotopes of these substances were injected into taurine-depleted and control cats. Sulfate did not serve as a precursor for taurine synthesis, whereas cystine underwent only a moderate conversion to taurine. This is in keeping with the low level of cysteinesulfinic acid (CSA) decarboxylase activity in cat liver. There was no difference between the activity of CSA decarboxylase in tissues from control cats and that in tissues from taurine-depleted cats. The pattern of tissue accumulation of [35S]taurine and from [35S]cystine also indicated that tissues from taurine-depleted cats do not synthesize [35S]taurine more rapidly than tissues from control cats. The data did not indicate a difference in taurine uptake by tissues of control and deficient cats, but progressive accumulation in deficient cats suggested that the turnover rate of taurine is decreased by the deficiency. Since supplementation of the purified diet with cysteine has been found previously to be inadequate to prevent progressive taurine depletion of the retina and its subsequent degeneration and since conversion of sulfur compounds to taurine in vivo is inadequate, taurine can be considered an essential nutrient for the cat.