In a study of the neural processes that mediate visual attention in humans, 32-channel recordings of event-related potentials were obtained from 14 normal subjects while they performed a spatial attention task. The generator locations of the early C1, P1, and N1 components of the visual evoked response were estimated by means of topographic maps of voltage and current source density in conjunction with dipole modelling. The topography of the C1 component (ca. 85 ms post-stimulus) was consistent with a generator in striate cortex, and this component was unaffected by attention. In contrast, the P1 and N1 components (ca. 95 and 170 ms) exhibited current density foci at scalp sites overlying lateral extrastriate cortex and were larger for attended stimuli than for unattended stimuli. The voltage topographies in the 75-175 ms latency range were modeled with a 5-dipole configuration consisting of a single striate dipole and left-right pairs of dipoles located in lateral extrastriate and inferior occipito-temporal areas. This model was found to account for the voltage topographies produced by both attended and unattended stimuli with low residual variance. These results support the proposal that visual-spatial attention modulates neural activity in extrastriate visual cortex but does not affect the initial evoked response in striate cortex.