Between 20 and 30% of AIDS patients have neurological symptoms characterized by motor impairment, memory loss and progressive dementia. Previous studies have implicated the HIV derived gp120, which produces behavioral deficits and electrophysiological alterations in rats. The goal of the present study was to describe the effect of this protein on the P3 event-related potential (ERP), evoked by a passive discrimination task in rats. We used II rats divided into two groups: HIV gp120 (n = 6) and control (n = 5). We recorded the P3 wave before any treatment (baseline), during the i.c.v. administration of either HIVgp 120 (700 ng/5 days) or saline (pH 7.2), and 24 h, 7, 14 and 21 days after the last injection. There were no changes between groups in the amplitude or latencies of the observed components (N1, P2, N2 and P3) evoked by target stimuli, during baseline or during the injection period. However, the HIV gp120 group showed a significant amplitude reduction in P3 wave 24 h after the last injection, while the N1, P2 and N2 waves remained unchanged. However, from the 7th day through the 21st day, P2 and N2 components also disappeared and only the N1 component could be observed in the HIV gp 20-treated group. These changes in the N2, P2 and P3 potentials, suggesting an alteration in cognitive processes, further support the neurotoxic activity of HIV gp120 and its role in AIDS dementia.