Event-related potentials (ERPs) were recorded in two experiments involving selective visual processing. In Experiment 1, subjects attended to light flashes emanating from one visual field, in order to detect occasional slightly deviant "targets", while ignoring equiprobable stimuli from the opposite field. ERPs elicited by stimuli in an attended field were characterised by larger posteriorly distributed P120 and N170 components, and a larger anteriorly distributed N145 component. In addition, these ERPs were, in comparison to those elicited by unattended stimuli, more negative-going in the latency region of approx. 200-400 msec. This late effect had a marked fronto-central distribution. In Experiment 2 subjects attended to either horizontal or vertical bars, displayed equiprobably in the same spatial location. No enhancement of early components was observed as a function of attention but, as in Experiment 1, a late, sustained, fronto-centrally distributed negative shift was observed in ERPs elicited by "attended" compared to "unattended" stimuli. It was concluded that the enhancement of P120 (P1) observed in Experiment 1 reflects the engagement of attentional mechanisms specific to the selection of stimuli on the basis of spatial cues. The later sustained negative shift seen in both experiments was considered to reflect a feature of within-channel processing common to both spatial and non-spatial selective tasks.