Quantitative and qualitative chromatographic analyses were made of the radioiodinated compounds found in the thyroids of Antrozous pallidus 24 hours after injection of 131I into a group held at 22° C and into other groups held for varying periods at 4° C. The results indicate that the thyroid of the hibernating bat is still active in the synthesis of iodinated amino acids through the sixth day at an ambient temperature of 4° C. The quantity of each compound regresses with time in hibernation, as does the uptake of iodine. Among the compounds containing organically bound 131I, diiodotyrosine becomes progressively less prominent while monoiodotyrosine and thyroxine become progressively more prominent with time in hibernation.