During a 1-sec tachistoscopic exposure, Ss responded with a right or left leverpress to a single target letter from the sets H and K or S and C. The target always appeared directly above the fixation cross. Experimentally varied were the types of noise letters (response compatible or incompatible) flanking the target and the spacing between the letters in the display. In all noise conditions, reaction time (RT) decreased as between-letter spacing increased. However, noise letters of the opposite response set were found to impair RT significantly more than same response set noise, while mixed noise letters belonging to neither set but having set-related features produced intermediate impairment. Differences between two target-alone control conditions, one presented intermixed with noise-condition trials and one presented separately in blocks, gave evidence of a preparatory set on the part of Ss to inhibit responses to the noise letters. It was concluded that S cannot prevent processing of noise letters occurring within about 1 deg of the target due to the nature of processing channel capacity and must inhibit his response until he is able to discriminate exactly which letter is in the target position. This discrimination is more difficult and time consuming at closer spacings, and inhibition is more difficult when noise letters indicate the opposite response from the targe