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During amphibian metamorphosis dramatic changes occur in the morphogenesis and differentiation of the epidermis. Concurrently with these changes, the 63 kDa keratin gene is upregulated from low basal levels to high levels. What makes these processes unique is that they are controlled by triiodothyronine (T3) and can be duplicated in cultures of purified epidermal cells. Since there is a 2 day lag period between the addition of T3 and the upregulation of keratin gene expression and terminal differentiation, recent studies have focused on identifying the genes activated during the lag period. We assume that the transcription factors required for upregulation of the keratin gene are induced by T3 during the lag period, and therefore we have cloned the keratin gene so that promoter analyses can be conducted. S1 mapping assays have shown that the same transcription start sites are used during premetamorphosis when the keratin gene is basally expressed, during metamorphosis when it is T3-upregulated, and in the adult epidermis where it is expressed independently of T3. During the early part of the lag period TR beta and AP-2 mRNA levels are upregulated in the epidermis by T3. The transcription factor AP-2 is expressed at high levels in the skin of premetamorphic larvae and induced about fivefold by T3 but is not induced in an epithelial cell line (XL-177). Since the keratin mRNA, AP-2 mRNA, and other genes induced during the lag period are expressed in premetamorphic larvae it appears that T3 functions by upregulating the expression of genes previously activated by a T3-independent process. This preprogramming may account for the tissue specificity of T3 action during metamorphosis.