The ability of an extracorporeal hemoperfusion system employing neutral Amberlite resin to bind thyroid hormone and to decrease circulating levels of triiodothyronine (T3), thyroxine (T4), and free thyroxine (FT4) was evaluated in dogs made thyrotoxic by the intramuscular administration of thyroid hormone. Since the resin column and tubing were charged with saline, the effects of hemodilution from this source on serum T3 and T4 was assessed by control perfusion through a column which did not contain any resin. After correction for hemodilution, the mean serum T3, T4 and FT4 decreased during 2 hours of resin hemoperfusion by 39%, 35%, and 46%, respectively. Hormonal clearance rates were calculated in two experiments and the estimated net hormone removed averaged 60.4 mug of T3 and 1990 mug of T4. Hematologic indices and routine chemistries did not change significantly in these dogs during the procedure except for a decrease in mean serum albumin concentration and an increase in mean serum glucose concentration. Hemoperfusion through this resin system seems to be a safe, effective means of decreasing serum T3, T4, and FT4 in thyrotoxic dogs and warrants evaluation for the treatment of thyroid storm in man.