We examined the tension (force) responses induced by rapid temperature jumps (T-jumps) in electrically stimulated, intact fibre bundles (5-10 fibres, fibre length ~2 mm) isolated from a foot muscle (flexor hallucis brevis) of the rat; the muscle contains ~90 % type 2 fast fibres. In steady state experiments, the temperature dependence of the twitch tension was basically similar to that previously described from other fast muscles; the tetanic tension increased 3- to 4-fold in raising the temperature from ~2 to 35 °C and the relation between the tetanic tension and the reciprocal absolute temperature was sigmoidal with half-maximal tension at 9.5 °C. A rapid T-jump of 3-5 °C was induced during a contraction by applying an infrared laser pulse ( = 1.32 µ, 0.2 ms) to the 50 µl trough containing the fibre bundle immersed in physiological saline. At ~10 °C, a T-jump induced a large transient tension rise when applied during the rising phase of a twitch contraction, the amplitude of which decreased when the T-jump was delayed with respect to the stimulus; a T-jump probably perturbs an early step in excitation-contraction coupling. No transient increase was seen when a T-jump was applied during twitch relaxation. A T-jump applied during the plateau of a tetanic contraction induced a tension rise to a higher steady tension level; the tension rise after a T-jump was 2-3 times faster than the corresponding phase of the initial tension rise in a tetanus. The approach to a new steady tension level after a T-jump was biphasic with a fast (phase 2b, ~35 s^-1 at 10 °C) and a slow component (phase 3, < 10 s^-1). The rates of both components increased (Q₁₀ ~3) but their amplitudes decreased with increase of the steady temperature. These results from tetanised intact fibres are consistent with the thesis previously proposed for Ca²⁺-activated skinned fibres, that the elementary force generation step in muscle is enhanced by increased temperature; the findings indicate that an endothermic molecular step underlies muscle force generation.