The nucleus basalis of Meynert (nbM, substantia innominata) in the basal forebrain provides a single major source of cholinergic innervation for the entire cerebral cortex. We tested the effects of nbM lesions on rabbit P300-like potentials. The P300-like event-related potential (ERP) was recorded in 14 female adult white rabbits using a conventional auditory oddball paradigm. The probability of occurrence for the 2-kHz and 1-kHz stimulus tones was 90% (frequent) and 10% (rare), respectively. The nbM was destroyed bilaterally in seven rabbits referred to as the nbM (+) group. In the other seven rabbits [nbM (-) group], putamen nuclei (n=6) or amygdaloid nuclei (n=1) were destroyed bilaterally. The evoked responses were recorded before and 1 week after the destruction. In the nbM (+) group, P300 amplitude to rare stimuli significantly decreased after the lesion. In the nbM (-) group, no component of ERPs showed changes after the lesions. These results indicate that the nbM might be involved in the generation of the rabbit P300.