Comparative studies of x-radiation effects on both isolated cold blooded (frog) and intact warm blooded (rabbit) muscles were performed. Frog muscles irradiated with doses above 50 kr showed early fatigue, contracture, prolongation of relaxation time, decreased contraction amplitude for heavy loads, and histologic changes noticeable 8 hours after exposure. Rabbit muscles exposed to 72 kr exhibited a gradually progressing impairment of function. Complete abolishment of function was reached within 24 hours following irradiation and was accompanied by severe histologic alterations.