Event-related potentials were recorded from eight subjects during simple reaction task and two visual discrimination tasks. Stimuli were horizontal arrays of five different alphabets. In two discrimination tasks, subjects were required to respond to target with one hand, and to non-target with another hand. In easy discrimination task, arrays that contained letter 'A' were designated as targets. Arrays containing any of 'A'/'B'/'C' letters were the targets in difficult discrimination task. In difference waves obtained by subtracting simple reaction ERPs from discrimination ERPs, negative deflections, which were termed NA by Ritter et al. (1983), were observed. This negativity became longer in duration when discrimination task was more difficult, but the scalp distribution of later portion of this negativity was different from those in Ritter et al. (1988)'s experiment. Prolongation of NA in the present experiment was probably caused by overlap of search negativity (Okita et al., 1985) on NA. The process which is reflected in later deflection of NA is not memory search as Ritter et al. (1988) interpreted, but is likely spatial search for multi-element displayed simultaneously.