The effects of serotonergic and dopaminergic depletions on event-related potentials (ERPs) generated by an auditory "oddball" paradigm were evaluated. Eighteen rats received either sham or six-hydroxydopamine (6-OHDA) lesions to the ventral tegmental area (VTA), and were subsequently implanted with electrodes in the frontal cortex, dorsal hippocampus (DHPC), and amygdala (AMYG). In these animals, a series of large amplitude potentials in the 10-200 ms latency range could be recorded from all the brain areas tested. In addition, late positivities (in the 300-400 ms range) were identified in DHPC and AMYG. 6-OHDA lesions to the VTA were found to produce a 30-46% reduction in dopamine, but did not significantly alter any of the ERP components. A second series of rats were implanted with electrodes in cortex and DHPC. These rats then received vehicle injections and subsequently injections of parachlorophenylalanine (PCPA). PCPA produced a 50% depletion of serotonin concomitant with significant reductions in the negative components in the 50-100 ms range recorded in hippocampus and cortex. These studies support a role for serotonin but not dopamine in the processing of passively presented auditory stimuli and further suggest that the rat may be a good model for the exploration of long latency ERPs.