Event-related brain potentials (ERPs) associated with semantic relation were recorded from twelve subjects while detecting nonwords preceded by a specific word in a series of successively presented words and nonwords. In this task, subjects were required the lexical decision for probes preceded by the specific prime (TASK RELEVANT condition), but not for the probes preceded by other primes (TASK IRELEVANT condition). The amplitude of a negative ERP component (N400) was significantly attenuated when the probes were preceded by semantically related primes. This attenuation was greater in TASK RELEVANT condition than TASK IRELEVANT condition, regardless of the modulation of such components overlapping on N400 as CNV resolution and late positive component in TASK RELEVANT condition. The advantages and disadvantages for imposing any task on probes were discussed.