The modulation of hepatic and renal cysteine sulfinic acid decarboxylase (EC 4.1.1.29) activities by triiodothyronine (T 3) was studied in a series of experiments. In a dose-response study, hepatic cysteine sulfinic acid decarboxylase activity (CSAD) was depressed by 65% and renal activity was increased threefold in rats injected with 100 μg T 3/100 g body wt for 7 days when compared to rats injected with 0.3 μg T 3/100 g body wt. Western blot analysis indicated that these changes in CSAD activity were due to changes in the quantity of CSAD protein. Changes in hepatic and renal activities were not evident until 24 h after T 3 administration. In response to T 3 clearance, hepatic and renal CSAD activities approached euthyroid values 4–7 days after cessation of T 3 injections although serum T 3 concentrations were no different from euthyroid values 48 h after T 3 injections were stopped. These data indicate that thyroid hormone effects persist after T 3 clearance. The response of CSAD to thyroid status may be related to its role in taurine biosynthesis.