The ultrastructure of myelinated fibres in the ventro-medial area of medulla oblongata of metamorphosing Bufo bufo was studied. In the first stages of metamorphosis the fibres show normal distribution of the axonal organelles, while in the myelin sheaths the lamellae appear detached from one another with an irregular period and absence of the intra-period line. In the middle and final stages of metamorphosis there are myelin sheaths like those described above, but there are others with compact lamellae, regular period and intraperiod line always visible. Moreover, it is possible to observe myelin sheaths with intermediate characteristics. In that period of metamorphosis many myelinized axons may show degenerative pictures, more or less evident. However, these always precede possible degenerative aspects of the myelin sheath. The different morphological characteristics of the sheaths can be interpreted as different maturation stages of the myelin sheath, from embryonal to definitive-conditions. It is less easy to interpret the significance of the numerous degenerating nervous fibres observed in the cone sidered area of medulla oblongata. On the basis of the bibliographic data and preceding studies, the Authors think that degeneration of the fibres is not necessarily linked to cellular degeneration, but to a quiescent condition in which many nervous cells of the Amphibian's CNS would settle during metamorphosis while waiting for replacement of the target organs.