Transformation of skin from larval to adult types in a salamander, Hynobius retardatus, which had been reported to show neotenic reproduction in a specific environment, was examined morphologically in normally metamorphosing, precociously metamorphosing and metamorphosis-arrested larvae. Typical larval skin was composed of an epidermis constituted by three types of cells such as apical, Leydig, and basal cells. The Leydig cells were larval specific, and thus disappeared and were replaced by adult epidermal cells during the metamorphosis. Disappearance of the Leydig cells was accomplished by apoptosis as confirmed by the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick-end-labeling method and electron microscopy during the normal metamorphosis and precocious metamorphosis induced by exogenously applied triiodothyronine. Typical adult skin was composed of epidermis constituted by stratified squamous cells and of dermis mainly occupied with two types of dermal glands, mucous and serous glands. When the metamorphosis was arrested by different procedures (thyroidectomy, hypophysectomy, goitrogen treatment, and rearing at low temperature), the larval-specific Leydig cells fully remained in the epidermis, suggesting that the disappearance of these depended on the thyroid activity. Contrary to this, dermal glands behaved differently from the Leydig cells, though they developed and differentiated from epidermal basal cells and constituted the same skin. Those in the metamorphosis-arrested (thyroidectomized, hypophysectomized, or goitrogen-treated) larvae, except in the larvae reared at 4 degrees C, appeared a little later than in the controls. Thus, the aged, metamorphosis-arrested larvae had skin which consisted of larval type epidermis (Leydig cells) and adult type dermis (mucous and serous glands).