The aim of this work was to study the time-course of preparation when a two choice stimulus arrives, in a predictable (Experiment I) or unpredictable (Experiment 11) manner, after the moment at which it was expected. Subjects watched the screen of an oscilloscope on which a spot moved horizontaly. Vertical lines marked (a) the starting point of the spot's course, t o , and (b) the points in time, tl . . . ti, at which the stimulus could arrive. I n Experiment I five intervals between tl and ti, ranging from I 50 to 900 ms were used in different blocks of trials. I n Experiment I1 three tl-t, intervals, ranging from IOO to 400 ms mixed in a random way were used. In both cases subjects were asked to react as fast as possible when the stimulus was presented at moment tl. The time course of preparation was studied when the stimulus arrived at moment(s) ti. The results show that reaction time reaches a maximum 250 ms after tl, irrespectively the predictability of the tl-ti interval. It is also shown that an interval of about I s was necessary to obtain reaction times as rapid as those observed at moment tl. The results are discussed in terms of the evolution of an hypothetical "state of preparation" after a maximum which remains unused.