The changes of myosin isoform pattern and of its associated light chains in relation to the myosin ATPase profile were analysed in different muscles of the hypothyroidian amphibian Pleurodeles waltlii submitted to terrestrial stepping, using electrophoretic and histochemical techniques. These changes were specific to the muscle type but appeared globally characterized by a type-IIB to type-IIA/I fibre transition associated with a transition from fast to intermediate and/or slow myosin isoforms. These results are similar to the effects of endurance training on locomotor muscles of mammals. The diaphragm of experimental animals was also characterized by a complete disappearance of the larval myosin isoforms which were detected in the diaphragm of control animals. The myosin pattern of ventricular muscle did not change following terrestrial stepping. This work indicates that thyroid hormone does not regulate the muscle adaptations that occur following terrestrial stepping and suggests a more complex mechanism of regulation in which innervation could be implicated.