Tension responses due to small, rapid length changes (completed within 40 microseconds) were obtained from skinned single frog muscle fiber segments (4-10 mm length) incubated in relaxing and rigor solutions at various ionic strengths. The first 2 ms of these responses can be described with a linear model in which the fiber is regarded as a rod, composed of infinitesimally small, identical segments, containing one undamped elastic element and two or three damped elastic elements and a mass in series. Rigor stiffness changed less than 10% in a limited range, 40-160 mM, of ionic strength conditions. Equatorial x-ray diffraction patterns show a similar finding for the filament spacing and intensity ratio I(11)/I(10). Relaxed fibers became stiffer under low ionic strength conditions. This stiffness increment can be correlated with a decreasing filament spacing and (an increased number of) weakly attached cross-bridges. Under low ionic strength conditions an additional recovery (1 ms time constant) became noticeable which might reflect characteristics of weakly attached cross-bridges.