I131i labeled analogues were studied using paper electrophoresis in Veronal ;barbital) and tris-hydroxymethylaminomethane buffers utilizing regular and reverse flow. The binding of D-thyroxine by serum protein was similar to that of L-thyroxine. The affinity of L-triiodothyronine for thyroxine-binding globulin was much weaker than L-thyroxine. Tetraiodothyro-acetic and -propionic acids were associated with 3 components of the serum in the veronal buffer. The greatest part was attached to pre-albumin, most of the remainder was in albumin, and a small quantity was found in the thyroxine-binding globulin. In tris buffer the major portion of these acids was attached to the pre-albumin. The pre-albumin position of these analogues is due to protein binding.