Studied features of functioning of nervous - muscular system at white rats of two age groups: preadolescent (5 weeks) and pubiral (24 weeks), in conditions expe- rimental hyperthyroidism (HT). It is established, that in conditions HT at action of the raised concentration thyroxine characteristics of excitation gastrocnemius muscles essentially changed at irritation of a sciatic nerve in groups preadolescent and puberal animals. In all age groups in conditions HT increase of a thres- hold of excitation gastrocnemius muscles is marked at indirect stimulation and decrease at direct sti- mulation; also in all age groups in conditions HT reduction of time chronaxy muscles is fixed, both at direct, and at indirect irritation. At preadolescent animals, as against puberal in conditions HT at action of the raised concentration thyroxine on nervous - muscular system it is not revealed authentic change of the latent period and amplitude of potential of action (PA). The conclusion is made, that in con- ditions HT change of a threshold of excitation and chronaxy gastrocnemius muscles both at direct, and at indirect irritation do not carry age specificity and have an identical orientation, both at preadolescent, and at puberal rats. At preadolescent animals in conditions HT, as against puberal the parameter of amplitude and latent period PA authentically did not change, that can testify to smaller sensitivity of the caused answers gastrocnemius muscles to the raised concentration thyroxine, probably, by virtue of imma- turity peripheral neuromotor the device.