We investigated whether the potential corresponding to the human P3b could be recorded on the dura mater over the frontal cortex of the rat in an active discrimination task. Rats were trained to press a bar within 1200 msec after cessation of the target tone (1000 Hz) lasting for 800 msec, and to withhold an overt response to the standard tone (2000 Hz). Rats were given intracranial electrical stimulation to the medial forebrain bundle as a reward only when they correctly responded to the target tone. The stimulus probability of the target tone was manipulated at 3 levels: 30, 50 and 70% of all trials. Large, slow positive deflections with peak latency at about 400-500 msec were elicited to the target tone, which were preceded by a large negative potential with its peak around 180 msec, whereas such a clear deflection was not elicited to the standard tone irrespective of the stimulus probability. This positive slow deflection was very similar in morphology to the human P3, and this P3-like potential was significantly larger when the target tone was less frequent (30%) than in the case of more frequent targets (50 and 70%). These results suggest that a potential highly similar to P3b can be recorded in the rat, and that it may provide a useful model to investigate the neurophysiological basis of the human P3b.