Previous research has shown that a startle ‘go’ stimulus, presented at a constant latency with respect to a warning stimulus, is capable of eliciting an intended voluntary movement in a simple reaction time (RT) task at very short latencies without involvement of the cerebral cortex (Carlsen et al. in Exp Brain Res 152:510-518, 2003; J Motor Behav 36:253-264, 2004a; Exp Brain Res 159:301-309 2004b; Valls-Solé et al. in J Physiol 516:931-938, 1999). The purpose of the present experiment was to determine the effect of temporal uncertainty on response latency during an RT task that comprised a startle stimulus. Participants were required to perform an active 20° wrist extension movement in response to an auditory tone that was presented 2,500 to 5,500 ms after a warning stimulus, in 1,000 ms increments. On certain trials the control auditory stimulus (80 dB) was unexpectedly replaced by the startle stimulus (124 dB). When participants were startled the intended voluntary movement was initiated at approximately 70 ms, regardless of foreperiod duration. The magnitude and invariance of response latencies to the startle stimulus suggest that the intended movement had indeed been prepared prior to the arrival of the imperative go stimulus, within 2.5 s of the warning stimulus. Furthermore, there was no evidence that the prepared movement decayed over a period of at least 3 s.