Forty cats with hyperthyroidism were treated using 200 to 300 (typically 250) MBq [corrected] of orally administered 131I. Thirty-six cases (90%) were successfully treated, as assessed by resolution of clinical signs and reduction in plasma thyroxine concentrations to normal or reduced values after treatment. Although higher doses of 131I appear to be required when the radioisotope is administered orally rather than intravenously, a less stressful administration procedure and greater availability of therapy capsules offer useful advantages for treating thyrotoxic cats.