We investigated the presence and the characteristics of the Simon effect for onset and offset targets when these stimuli are randomly intermixed. In Experiment 1, two possible target locations were occupied by an occluder. On onset trials, a target appeared above an occluder, while on offset trials one of the occluders disappeared, revealing the target underneath. In Experiment 2, four stimuli appeared randomly in six possible locations. On onset trials, a new stimulus appeared in an empty location while on offset trials, one of the initial stimuli disappeared. In both experiments, the Simon effect for onset and offset targets was characterized by similar size, time course and sequential modulation, suggesting similar sensorimotor interactions between target and response locations. However, the Simon effect in the current trials was more evident when the same type of target (onset or offset) was repeated on successive trials demonstrating the role of stimulus category in its modulation. (Contains 7 figures.)