A study was carried out to investigate the effects of ionic strength and monovalent cations on isometric, Ca2+-activated force and rigor responses in mechanically skinned muscle fibres. Three types of skeletal muscle fibres were used: rat fast- and slow-twitch fibres and toad twitch fibres. The contractile apparatus of rat slow-twitch fibres was affected differently from that of rat fast-twitch and amphibian twitch fibres when changing the ionic strength (expressed either in terms of ionic equivalents as I or formally as gamma/2) and [K+]. Thus, the apparent sensitivity to Ca2+ decreased substantially more in slow-twitch fibres (by a factor of 20) than in the other fibre types (by a factor of 12) when I and [K+] were increased from 94 to 354 mM and from 56 to 316 mM respectively. Maximum Ca2+-activated force, however, declined only by a factor of 2.2 in slow-twitch fibres compared with 4.2 in the other fibre types, when I was increased from 154 to 354 mM. In slow-twitch fibres the force oscillations of myofibrillar origin were found to increase substantially in amplitude, duration and frequency at low values of I and almost disappeared at high ionic strength. At low values of I, it was also discovered that ca. 50% of the fast-twitch fibres responded with myofibrillar force oscillations when submaximally activated. The characteristics of these oscillations were different from those of slow-twitch fibres. Rigor force levels were found to decline markedly with increasing iota and [K+] in all fibre types. Unexpectedly, once rigor force was established in a certain ionic environment, the level of force was stable regardless of further changes in ionic strength and monovalent cation concentration. These results indicate that the rigor cross-bridges can be formed in different stable positions and that the probability of attachment in certain positions (rather than the total number of cross-bridges that can be formed) is influenced by the ionic conditions. Further experimental evidence provided in this study shows that the increase in [K+] is mainly responsible for the decrease of the Ca2+-sensitivity of the contractile apparatus and that ionic strength (expressed as I rather than gamma/2) influences markedly the maximal Ca2+-activated force, the maximum steepness of the pCa-force relations and the oscillatory processes of myofibrillar origin.