Abstract.Zinc‐induced relaxation in glycerol‐extracted rabbit psoas fibre bundles was studied under strictly iso‐tonic and isometric conditions and compared with the effects of zinc on the visco‐elastic behaviour of resting fibre bundles subjected to quick stretch. 0.a mM zinc + 1 mM magnesium + 0. 4 mM ATP does not affect the visco‐elastic properties but induces relaxation of isometrically and isotonically contracted fibre bundles. Isotonic relaxation is complete only when the previous shortening does not exceed 25 percent of initial fibre length; the relaxing effect is practically absent when the bundle has shortened about 75 percent of its initial length. 2 mM zinc + 1 mM magnesium + 6 mM ATP and 4 mM EDTA + 4 mM magnesium + 4 mM ATP, which markedly decrease the visco‐elastic resistance in the fibre bundle produces a pronounced isotonic relaxation even after extreme shortening of the bundles. Thus, the isotonic relaxation of glycerol extracted fibre bundles is dependent upon a decrease in the visco‐elastic resistance. The isometric relaxation, on the other hand, does not need any pronounced visco