Isoforms of myosin heavy chain and tropomyosin convert during metamorphosis of Xenopus laevis with larval-to-adult remodeling of dorsal muscle (Nishikawa and Hayashi, 1994 , Dev. Biol. 165: 86-94). In the present study, other markers for muscle remodeling during metamorphosis were determined by SDS-PAGE analysis. The amounts of twelve muscle proteins changed remarkably during metamorphosis. Among these, a 54-kDa molecule was found to be desmin, and the relative content/total proteins decreased dramatically through metamorphosis. In hindlimb muscle, desmin content increased fourfold during prometamorphosis, when myoblast proliferation and fusion occurred. With further myotube maturation, this content decreased by 1/2 while that of muscle actin continued to increase. Thus, desmin up- and down-regulation in hindlimbs mark early and late phases of myogenesis, respectively. In tall muscle, the desmin content decreased continuously to (1/8) before and during metamorphosis, due to tall muscle growth and maturation. In dorsal muscle, three desmin changes occurred: a pre-metamorphic decrease, a transient increase at prometamorphosis, and a rapid decrease at the climax stage. Immunohistochemical analysis showed desmin+ cells to be present between young (adult-type) myotubes and replicating (PCNA+) cells in dorsal muscles, correlating the transient desmin upregulation in dorsal muscle with the initiation of adult-type myogenesis. After the upregulation, dorsal muscle desmin decreased to (1/8). This rapid down-regulation was replicated by administration of triiodothyronine (T3) to tadpoles, suggesting a significant role for T3 in dorsal muscle remodeling during metamorphosis. Collectively, these results show that analysis of desmin expression and PCNA-immunohistochemistry are good tools for determining the sites and timing of larval-to-adult muscle remodeling during Xenopus laevis metamorphosis.