The study examined the effects of task relevancy upon visual ERPs elicited by occasionally presented (15% for each) nonwords (NW) and semantically deviant (SD) words, which were embedded among frequent (70%) words belonging to the same semantic category (C). Twelve students performed four tasks; detection of NW, detection of SD, detection of both NW and SD, and word/nonword-choice. The effect of task relevancy on the N400 to nonwords was only being overlapped by the parietal P3. In contrast, the N400 to SD words enlarged and was followed by the P3 when they became targets. Nonwords elicited larger N400 than SD words when both of them were not targets. Both rare stimuli elicited similar N400 when they became targets, and it was also the case when subjects responded to both NW and SD words in one task. Since the ERPs in the word/nonword-choice task were similar to those in the nonword-detection, the effects were not caused by a motor response itself. DESCRIPTORS: Event-related potentials, N400, Task relevancy, Semantic categorization.