Endogenous components of evoked potentials resembling P300 in humans were sequentially studied in 3 cynomolgus monkeys (Macaca fascicularis) using an auditory 'oddball' paradigm. The two different auditory stimuli were 500 Hz and 4000 Hz tones, designated as the 'frequent' and 'rare' stimuli, respectively. The probability of 'rare' tone presentation was initially 0.2. We further used probabilities of 0.1, 0.3 and 0.5. The 'rare' stimulus was reinforced by electrical stimulation, which followed the onset of the high tone by 700 msec. After 3-5 training sessions, a late positive wave was observed following the 'rare' tone. The latency of this P300-like signal was 314 +/- 16.2 msec, and the amplitude was 23.6 +/- 3.14 microV. The amplitude of this potential was modified by changes in stimulus presentation probability and by withholding reinforcement.