The changes in compliance during the development of maximal isometric contraction were studied at 0 degree C in isolated frog (Rana temporaria) sartorii. Three types of method were compared: controlled release from the plateau of tetanic tension (P0); controlled release at various times during the development of tension; and evaluation of the elastic stretching by means of an additional compliance. The length variations were performed at constant velocity under the control of an electromagnetic ergometer. It was confirmed that muscular compliance decreased with increase in developed tension. Furthermore, for a given value of tension, the compliance determined during the elastic lengthening was found to be significantly higher than the compliance measured during the elastic shortening. It was concluded that the variation in compliance during stretching and releasing seems to be due to a difference in behaviour of the elastic material.