To support the idea that temporal information processing may depend on an internal clock, Treis-man et al. proposed a pacemaker model (Treisman, M., Faulkner, A., Naish, P.L.N., Brogan, D., 1990. The internal clock: Evidence for a temporal oscillator underlying time perception with some estimates of its characteristics frequency. Perception 19, 705-743.) and a technique for interfering with it by introducing an external periodic phenomenon. Experimental results obtained by these authors on time estimation and production tasks support this model. In another study, Treisman et al. established that the pacemaker also affects reaction times (RT) (Treisman, M., Faulkner, A., Naish, P.L.N., 1992. On the relation between time perception and the timing of motor action: Evidence for a temporal oscillator controlling the timing of movement. Quaterly Journal of Experimental Psychology 45A, 235-263.). In the present study, we addressed the question as to which information processing stage (Sanders, A.F., 1980. Stage analysis of reaction process, In: Stelmach, G.E., Requin, J. (Eds.). Tutorials in motor behavior. North-Holland, Amsterdam, pp. 331-354.) is affected by this internal clock. For this purpose, we used the Additive Factors Method (Sternberg, S., 1969. The discovery of processing stages: Extension of Donder's method. In: Koster, W.G. (Ed.). Attention and Performance II. Acta Psychologica 30, 276-315.). To vary sensorial processing time, we used two visual stimulus intensities. Stimulus response mapping was manipulated to enhance central processing time. To modify the duration of the motor stages, the two responses could be given by two fingers on the same hand (right ring vs. middle finger) or by two fingers of the different hands (right ring vs. left middle finger). Intensity of the stimulus, stimulus-response mapping, and repertoire of responses were found to be additive. We obtained RT modulations similar to those obtained by Treisman et al. in 1992. No first order interactions were observed between the periodical phenomenon and the other manipulated factors but only a third order one. Two possible interpretations of these results are proposed.