Event-related potentials were studied while subjects performed physical and semantic discrimination tasks. Two negative components, NA and N2, were observed in both kinds of discriminations. The earlier component, NA, had a constant onset latency, but its peak latency varied as a function of stimulus complexity. N2 latency varied in relation to changes in the peak of NA. RT and P3 followed N2 by similar amounts of time across tasks. The NA and N2 components were interpreted as reflecting partially overlapping sequential stages of processing associated with pattern recognition and stimulus classification, respectively.