There are many higher-order cognitive processes whose neural mechanisms are hard to study by using nonhuman primates. The mental rotation task is one of the best studied of these. We investigated the spatiotemporal brain activity underlying mental rotation of different kinds of stimuli by measuring the magnetoencephalogram of subjects performing two kinds of mental rotation tasks. Visual stimuli in one experiment consisted of hand shapes presented at various orientations, and those in the other consisted of a set of alphabetic characters and their mirror images presented at various orientations. All stimuli were presented in the left visual field. Activity associated with visual stimulus processing was estimated in the lateral occipital lobe, basal occipitotemporal area, and inferior temporal gyrus. Activities related to higher visual processing were differed between the two kinds of stimuli we used. For the hand shape experiment, we found right inferior parietal lobule activity at 200-300 ms after the stimulus presentation. For the alphabetic characters, activity was found in the left superior temporal region at about 300 ms. Furthermore, activity related to mental stimulation in the inferior parietal lobule and premotor area were seen in the both experiments. In the experiments with hands, the premotor activity showed left-hemispheric dominance. However, in the experiments with alphabetic characters, the laterality of the premotor activity did not show left-hemispheric dominance. We inferred this difference was influenced by the subjects' familiarity with the mental rotation of visual stimuli.