Long-term enhancement of the evoked potential was induced in the primary somatosensory cortex of anaesthetized raccoons after mechanical stimulation of the skin was paired with electrical stimulation of the nucleus basalis of Meynert (NBM). Sets of 4 pulses, 0.5 ms duration at 300 Hz were delivered at 2-s intervals to the basal forebrain 80 ms before the glabrous skin on the 4th digit of the contralateral forepaw was stimulated mechanically. The average waveform of 30 evoked potentials was separated into an initial positive, a negative and a second positive component. During pairing of the skin and NBM stimuli, the area under the initial positive component was smaller than before or after pairing. The negative and second positive waves were unchanged. One minute after pairing, the initial positive wave returned to control values and continued to increase until the end of the experiment 50 min later, at which time it was 300% above control. The negative and second positive waves increased after the pairing to between 130 and 200% and remained at that level for the duration of the experiment. The effective NBM site for stimulation was the area rich in cholinergic neurons corresponding to the NBM. In control animals, repeated stimulation of the skin or NBM alone, or their random, unpaired stimulation together, did not enhance the somatosensory evoked potential. The results suggest that the NBM input enhances the efficacy of cortical responses to cutaneous input and thus may play a role in cortical neuronal plasticity.