The present study explored how response preparation, varied by relative response frequency, affects response conflict as expressed in the Simon effect. Previous studies showed that valid response cues, when presented before the imperative stimulus, increase rather than decrease the Simon effect. This finding was explained by the hypothesis that response cues trigger shifts of attention to the side of the prepared response, and that these attention shifts modulate processing of the imperative stimulus. We investigated whether cues are necessary for inducing shifts of attention and thereby modulating the Simon effect, or whether response preparation without cues is sufficient. In two experiments, participants performed a Simon task with one response being more frequent than the other. Results showed larger Simon effects for the more frequent (i.e. prepared) response than for the less frequent (i.e. unprepared) response. These results suggest that response preparation (rather than the cue that induces the preparation) triggers a shift of spatial attention which modulates the Simon effect.