In a parity judgment task, the ERPs at parietal electrode sites become the more negative the more mental rotation has to be executed. In two experiments, it was investigated whether a temporal relationship exists between the onset of this amplitude modulation and the moment when mental rotation is executed. Therefore, the duration of processing stages located before mental rotation was manipulated. The amplitude modulation was delayed when either the perceptual quality of the stimulus was reduced (Experiment 1) or when character discrimination was more difficult (Experiment 2). The results suggest that the onset of the rotation-related negativity might be used as a chronopsychophysiological marker for the onset of the cognitive process of mental rotation.