Activity of L-cysteinesulfinate carboxy-lyase (CSC) and persulfurase was measured in livers of rats fed 15% casein diets supplemented with (1) 0.53% cysteine, (2) 0.40% cysteine, (3) 0.40% cysteine + 0.10% sulfate, (4) 0.42% sulfate, or (5) 0.42% sulfate + 0.40% cysteine. The diets were fed to adult rats for 1 week and to weanling rats for periods of 1 and 4 weeks. At the end of each feeding period, the highest CSC activity was detected in livers from animals fed diet 4, but no differences in CSC activity were detected among livers from the other four groups. Persulfurase activity did not differ significantly among those weanling or adult rats fed the diets for 1 week but was lower among those fed diet 4 than diet 5 for 4 weeks. The acitivity patterns did not parallel those of taurine excretion or conjugation observed previously among rats fed the same diets, and may indicate that factors other than taurine synthesis and ingestion control those processes.