Event-related potentials (ERPs) were recorded from the scalp while subjects attended to sequences of bilaterally symmetrical arrays of 4 letters (2 in each visual half-field) that were flashed briefly at intervals of 280-520 msec. These sequences also included randomized presentations of unilateral 'probe' stimuli consisting of irrelevant bars (experiment 1) or potentially relevant letter pairs (experiment 2). The task was to pay attention to the letter pairs in either the left or the right half-field on a given run and to press a button when the two letters matched one another (targets). The ERPs to the bilateral arrays included an early positive wave (P1, peaking at 135 msec) that was enhanced over posterior scalp sites contralateral to the attended visual field. Both types of probe stimulus also elicited a larger early positivity in the P1 latency range (100-200 msec) when delivered to the attended half-field, followed in some cases by a more prolonged positive deflection. Notable for its absence was any sign of an enlarged posterior N1 component (160-200 msec), which was prominent in the ERP to attended-field stimuli in previous studies using randomized sequences of unilateral stimuli. Attended-field targets elicited large N2 and P3 (P300) components, which were greatly reduced or absent when targets occurred in the unattended field. The observed ERP effects were interpreted in terms of early sensory selection during visual spatial attention.