P300 latency is commonly thought to provide a chronometric
index of the duration of perceptual processing. Because
the evidence in favor of this assumption is controversial,
we examined whether P300 latency is influenced by perceptual
processes, response selection, and by motoric processes
in two experiments using a two-choice spatial stimulus-response
compatibility (SRC) task. Both experiments revealed additive
effects of perceptual difficulty with spatial SRC in reaction
time and P300 latency. In addition, Experiment 2 showed
that P300 latency measured in average waveforms is insensitive
to motoric processes. The influence of spatial SRC on P300
latency disagrees with the view that P300 latency is sensitive
only to stimulus evaluation processes. However, P300 latency
may be used to discriminate between influences on premotoric
and motoric processing stages. A response conflict account
for the SRC effect on P300 latency is suggested.