The Wisconsin Card Sorting Test (WCST) is generally regarded as the prototype of abstract reasoning task and has been routinely used to assess frontal lobe function in a variety of clinical and research contexts. However, there are growing concerns that the WCST fails to discriminate frontal patients from those with lesions in other brain regions or from normals. Event-related potentials (ERP) from frontal, fronto-temporal, temporal, parietal and occipital areas were recorded during the performance of a computerized version of the WCST in order to explore frontal versus non-frontal ERP indexes during WCST activation. The task protocol was contrived to focus on the differences between early and late trials of each WCST series. Cognitive processes underlying these two task conditions have been described as extradimensional and intradimensional shifts in attention, respectively. Differences between early and late WCST trials appeared as soon as 120 msec poststimulus and were associated with a negative field potential centred at the fronto-temporal region of the left hemisphere. Significantly larger amplitudes of the posterior P3b wave for late as compared with early WCST trials also lent support to claims of a strong involvement of working memory mechanisms during WCST performance. Results are discussed in terms of the implications for the utility of ERP measures in clinical neuropsychology.