In order to test the hypothesis that prestimulus alpha activity reflects top-down inhibitory processing, EEG was recorded from 16 subjects performing a color and a shape discrimination task. Both tasks required the inhibition of the task-irrelevant feature. Longer reaction times and P3 latencies showed that the shape task was more difficult than the color task. We suppose that these different task-difficulties are due to a higher salience of the color feature as compared to the shape feature. Interestingly, we observed significantly higher prestimulus total alpha activity in the shape task than the color task. We concluded that the inhibition of the more salient color feature in the shape task resulted in enhanced prestimulus alpha activity. Such a relationship between prestimulus alpha and poststimulus performance implies that prestimulus alpha reflects prestimulus top-down processing for preparing subsequent task-performance. Since we observed the 'task' effect of prestimulus alpha activity also in reaction times and in P3 latencies, prestimulus alpha seems to predict such poststimulus responses. Consequently, prestimulus ongoing alpha activity probably reflects top-down information and modulates subsequent poststimulus responses.