Event-related potentials(ERPs) were measured while subjects were constructing mental images of a letter of the alphabet. Following presentation of an uppercase letter, subjects were asked to form a mental image of the same letter in lower case, and determine whether or not it had an ascending or descending stroke (ascender or descender). ERPs were measured in a passive condition and then compared with the ERPs recorded while the subjects performed the discrimination task. During the discrimination task, negative potentials were observed in left frontal, central and parietal areas around 220 ms after the stimulus onset. These early negative potentials were dominant in the left hemisphere and are probably related to working memory processes in mental image formation.