THE duration of the foreperiod, that is, the interval between some warning signal and the stimulus, has long since been known as one of the main factors which influence the length of simple reaction time2–4. Klemmer1 has recently shown that reaction time is shorter with brief (0.25 sec.) constant foreperiods than with either longer constant foreperiods or variable ones. He suggested that the critical factor is ‘time uncertainty’, that is, uncertainty as to the moment of occurrence of the stimulus. This uncertainty would result from the length of the foreperiod as well as from its objective variability, due to the inaccuracy of internal time estimation processes. Speed of reaction clearly depends on the possibility for some transitory state of preparation to be reached at the right time.