The effects of a temperature jump (T-jump) from 5-7 degrees C to 26-33 degrees C were studied on tension and stiffness of glycerol-extracted fibers from rabbit psoas muscle in rigor and during maximal Ca2+ activation. The T-jump was initiated by passing an alternating current pulse (30 kHz, up to 2.5 kV, duration 0.2 ms) through a fiber suspended in air. In rigor the T-jump induces a drop of both tension and stiffness. During maximal activation, the immediate stiffness dropped by (4.4 +/- 1.6) x 10(-3)/1 degree C (mean + SD) in response to the T-jump, and this was followed by a monoexponential stiffness rise by a factor of 1.59 +/- 0.14 with a rate constant ks = 174 +/- 42 s-1 (mean +/- SD, n = 8). The data show that the fiber stiffness, determined by the cross-bridge elasticity, in both rigor and maximal activation is not rubber-like. In the activated fibers the T-jump induced a biexponential tension rise by a factor of 3.45 +/- 0.76 (mean +/- SD, n = 8) with the rate constants 500-1,000 s-1 for the first exponent and 167 +/- 39 s-1 (mean +/- SD, n = 8) for the second exponent. The data are in accordance with the assumption that the first phase of the tension transient after the T-jump is due to a force-generating step in the attached cross-bridges, whereas the second one is related to detachment and reattachment of cross-bridges.