In experiments on small bundles of intact fibers from a rat fast muscle, in vitro, we examined the decline in force in repeated tetanic contractions; the aim was to characterize the effect of shortening and of temperature on the initial phase of muscle fatigue. Short tetanic contractions were elicited at a control repetition-rate of 1/60 s and fatigue was induced by raising the rate to 1/5 s for 2-3 min, both in isometric mode (no shortening) and in shortening mode - in which each tetanic contraction included a ramp shortening at a standard velocity. In experiments at 20 °C (n=12), the force decline during a fatigue run was 25% in the isometric mode, but was significantly higher (35%) in the shortening mode. In experiments at different temperatures (10-30 °C, n=11), the tetanic frequency and duration were adjusted as appropriate and, for shortening mode, the velocity was adjusted for maximum power output. In isometric mode, fatigue of force was significantly less at 30 °C (~20%) than at 10 °C (~30%); the power output (force x velocity) was >10 x higher at 30 °C than at 10 °C and power decline during a fatigue run was less at 30 °C (~20-30%) than at 10 °C (~50%). The finding that the extent of fatigue is increased with shortening contractions and is lower at higher temperatures is consistent with the view that force-depression by inorganic phosphate, which accumulates within fibers during activity, may be a primary cause of initial muscle fatigue.