The biphasic late negative wave (Nx-Ny) evoked during the naming of pictures of objects was investigated in a series of experiments. The monitoring of several possible non-cerebral generators indicated that these components were probably cerebral in origin. The Ny wave differed in scalp distribution from the preceding CNV. It was unaffected by stimulus duration or by stimulus repetition. The Nx-Ny complex occurred in both a naming task and a mental rotation task. The slow waves following the biphasic negative wave differed significantly between these two tasks. In the mental rotation task, there was a large prolonged negativity in both parietal regions. The Nx wave (at approximately 250 msec) of the biphasic complex may represent the initial registration of the stimulus. The Ny wave (at approximately 420 msec) does not appear to be specifically related to semantic processing. Rather it may index some process involved in initiating the further evaluation of a complex stimulus. The slow waves that follow the biphasic negative complex may reflect the actual perceptual processing of the stimulus.