Propylthiouracil (PTU) and iodoacetate (IAc) partially inhibit the 5'-deiodination of 3,3',5'-triiodothyronine (rT3) in cerebral cortex, but PTU has no effect on the 5'-deiodination of thyroxine (T4). We now report that pretreatment of cerebral cortex microsomes with increasing concentrations of IAc or iodoacetamide inhibits the PTU-sensitive 5'-deiodination of rT3 with half-maximal inhibition at < 1 μM IAc. PTU-insensitive 5'-deiodination of rT3 and T4 requires 100-1000-fold higher concentrations of IAc to achieve similar inhibition. Trypsinization decreases both 5'-deiodinating activities by > 90%. These data suggest that enzyme SH groups do not participate in the 5'-deiodination of T4 or rT3 by the PTU-insensitive pathway. © 1983.