Averaged evoked potentials from primary somatosensory cortex (SEPs) were recorded before and after pairing the peripheral stimuli with electrical activation of the basal forebrain (BF) in anesthetized cats. Four pulses at 400 Hz were delivered to the BF 120 ms before each cutaneous stimulus and 10 to 660 such pairings were found to produce an enlargement of the SEP in 10 of 11 animals. The average increase in amplitude of the initial peak of the SEP was 69%. The SEP remained enhanced in five of six animals that were tested an hour or more after the pairing, and in one case the SEP was tested 4.5 h after pairing without diminution. The effective BF sites were located in the substantia innominata and at the rostral pole of the globus pallidus, regions known to contain many cholinergic cell bodies. Enhancement occurred consistently only if stimulation of the BF site elicited a positive wave in the cortex at a latency of 11 to 18 ms. Repeated BF stimulation without cutaneous input did not produce a change in subsequent SEPs. The long-term changes described here may be involved in experimentally- and naturally-induced cortical reorganization.