[Effect of L-thyroxine on skeletal muscle fibers of the frog].

L-thyroxin prevents neither the postdenervation decrease of resting MP, nor the increase of input resistance, nor the extrajunctional sensitivity to acetylcholine, but after the injection the cross-sectional areas of muscle fibers decrease more rapidly than after a mere denervation and muscle fibers of new type by myosin-ATPase activity do not appear. The same doses of hormone do not affect properties of muscle fiber membrane but increase the input resistance of membrane and decrease cross-sectional areas of muscle fibers. l-thyroxin seems unable to prevent the appearance of the postdenervation changes in skeletal muscle fibers in adult frog and does not take part in regulation of its differentiation.