Although the axolotl Ambystoma mexicanum is anatomically a larval amphibian, it does produce a small amount of endogenous thyroxine. According to the theory that different tissues have different thresholds of response to thyroxine, the axolotl may undergo certain biochemical metamorphic events without anatomical metamorphosis. Gel electrophoresis demonstrated that the hemoglobin and serum protein composition of the blood changed at approximately 125 and 210 days of normal development, respectively. Both events were prematurely inducible by thyroxine; metamorphosed animals had neotenous adult hemoglobin and serum protein electrophoretic patterns. On sodium dodecyl sulfate-polyacrylamide gels, denatured neotenous adult hemoglobin fractionated into components of 15,000 and 19,000, and 24,500 molecular weight (relative to that of bovine hemoglobin monomer, 15,500). Larval hemoglobin possessed components of 13,800 and 19,500 molecular weight. It was concluded that (1) at 100–150 days of normal development, the production of new hemoglobin polypeptide subunits was stimulated whereas the synthesis of larval subunits was inhibited, (2) the axolotl is not biochemically neotenous for at least two normally metamorphic events, and (3) the tissues responsible for certain biochemical metamorphic events probably have a lower threshold of response to thyroxine than the tissues responsible for anatomical metamorphosis.