The depressant effect of active shortening, previously established in intact muscle fibres, was studied during calcium induced contractures of chemically skinned fibres from the semitendinosus muscle of Rana temporaria and the psoas muscle of the mouse. The decrease in contractile activity was determined by comparing the rate of force redevelopment (at a given tension level) after a large (test) and a small (control) release step. Under standard experimental conditions (ionic strength: frog 135 mM, mouse 190 mM; Ca2+ 3.0 microM; Mg2+: frog 25 microM, mouse 100 microM; MgATP2-: frog 1.0 mM, mouse 2.0 mM) active shortening of 0.15 microns per sarcomere (in excess of control release) reduced the contractile activity by approximately 50% of the control in both frog and mouse muscle fibres. Full contractile activity was regained within less than 4 s during isometric activity after the shortening phase. The depressant effect of shortening was steadily reduced, to almost complete disappearance of the effect, by increasing the free calcium concentration within the range 1.5-12.0 microM. Similarly, an increase in ionic strength from 105 to 235 mM reduced the depressant effect by approximately 40%. In contrast, there was a progressive enhancement of the shortening effect as the magnesium ion concentration was increased from 25 to 590 microM. It is proposed that interaction between the myosin cross-bridges and the thin filament during sarcomere shortening leads to a decrease in troponin-calcium binding resulting in a temporary deactivation of the contractile system.