During the fixed foreperiod (4 sec) of two reaction time experiments, using 80 subjects, Hoffman reflexes were simultaneously evoked in both legs in a pseudorandom order at 13 different measuring points. The purpose of the study was to investigate the time course of amplitude changes in both legs, in muscles both involved and not involved in the response. In Experiment I, subjects responded after the imperative signal by a plantar flexion of either the right or left foot. Thus, reflexes were evoked via the motoneuron pool relevant for the response and via the contralateral irrelevant pool. In Experiment 2, subjects pressed a button with either the right or the left index finger. Thus, reflexes were evoked via motoneurons, irrelevant for the response. Reflex amplitudes during the foreperiod were larger than during the intertrial interval (ITI). An early increase was found at 100-200 msec after the warning signal. Thereafter, amplitudes decreased but remained larger than during the IT!. After 1,000 msec, a steady increase was found, with the largest amplitudes near the end of the foreperiod. Amplitude changes were not different for relevant and irrelevant muscles. It was concluded that the second increase might be part of a motor preparation process.Preparation for a movement can be investigated in reaction time experiments with a fixed interstimulus interval (lSI) between a warning stimulus (WS) and the response stimulus (RS). The use of WS causes a shortening of the mean reaction time (RT). The improved performance implies an earlier discharge of the motoneurons that innervate the muscles used in responding.The excitability of these motoneurons can be estimated by means of monosynaptic reflexes evoked via the same motoneuron pool before the movement is made (Gerilowsky & Tsekov, 1975; Requin, 1969). In general, changes in amplitude indicate changes in the output of the motoneuron pool, and consequently they are an index of fluctuations in (1) motoneuron excitability and (2) presynaptic inhibition of the la fibers, which conduct the afferent volley to the motoneuron pool (Figure 1). Evoking reflexes during an lSI provides the opportunity to study specific and nonspecific changes in the excitability of the motoneurons. Changes are specific when they are present exclusively in the muscle that is to respond. They are nonspecific when both involved and noninvolved muscles show the same reaction pattern. It is obvious that at a certain moment between stimulus and response, the specific activation has to be This work is part of a program suppor ted by the Dutch Organisation for the Advancement of Basic Research ZWO (Grant 58-44), the Dutch Health Organisation TNO (CLEO) , and the Esther Evrard Fund. The computer program for the LAB 8E was developed by Hans Adriaanse and Henk van Nieuwstadt. Lied Damman assisted in the experiments, which were carried out in the Dr. Hans Berger Clinic at Breda. Han Brunia and Loes Verhallen assisted in the data analysis.