The spatiotemporal analysis of brain activation during syllogistic reasoning, and the execution of 1 baseline task (BST) were performed in 14 healthy adult participants using high-density event-related brain potentials (ERPs). The following results were obtained: First, the valid syllogistic reasoning task (VSR) elicited a greater positive ERP deflection than the invalid syllogistic reasoning task (ISR) and BST between 300 and 400 ms after the onset of the minor premise. Dipole source analysis of the difference waves (VSR-BST and VSR-ISR) indicated that the positive components were localized in the vicinity of the occipito-temporal cortex, possibly related to visual premise processing. Second, VSR and ISR demonstrated greater negativity than BST developed at 600-700 ms. Dipole source analysis of difference waves (VSR-BST and ISR-BST) indicated that the negative components were mainly localized near the medial frontal cortex/the anterior cingulate cortex, possibly related to the manipulation and integration of premise information. Third, both VSR and ISR elicited a more positive ERP deflection than BST between 2500 and 3000 ms. Voltage maps of the difference waves (VSR-BST and VSR-ISR) demonstrated strong activity in the right frontal scalp regions. Results indicate that the reasoning tasks may require more mental effort to spatial processing of working memory.