Changes in the height of peak 2 obtained using linear sweep voltammetry and carbon paste electrodes chronically implanted in discrete brain regions of the unrestrained rat were measured under a variety of conditions; in the past this peak has been attributed to the oxidation of 5-hydroxyindoleacetic acid (5-HIAA). Unilateral 5,7-dihydroxytryptamine (5,7-DHT) lesions of the medial forebrain bundle reduced the 5-HIAA content of the striatum and hippocampus to 10% of the unlesioned side, but did not alter the height of peak 2 recorded in these regions. In contrast, microinfusion of uricase beside striatial electrodes reduced the height of peak 2 by 96%; systemic amphetamine-induced increases in the height of the peak were also prevented by this enzyme. These results indicate that uric acid, and not 5-HIAA, is mainly responsible for peak 2, and that changes in the height of this peak reflect changes in the extracellular concentration of uric acid.