Rat frontal cortex neurons exhibit alterations in firing in response to a 2 sec tone cue followed by rewarding medial forebrain bundle (MFB) stimulation. Nucleus basalis neurons supply up to 75% of the cortical cholinergic innervation. The nucleus basalis and ACh have been implicated as playing a role in cognitive function. Three experiments were designed to test the hypothesis that the nucleus basalis cholinergic system is involved in the generation of conditioned neuronal responses in the rat frontal cortex. Local microinjection of the cholinergic antagonist, atropine, into the frontal cortex suppressed the conditioned responses of 22 of 25 cortical single units. Unilateral kainic acid lesioning of the nucleus basalis resulted in a significant decrease in the proportion of units exhibiting conditioned responses in the cortex ipsilateral to the lesion (25%) compared to the proportion of responding units from the cortex of untreated animals (70%). When the firing rates of units encountered in the region of the nucleus basalis were monitored during presentation of the cue-MFB paradigm, 28 of 38 unit recordings exhibited significant increases or decreases in firing rate. Therefore, the results of the experiments indicate that the nucleus basalis cholinergic neurons are involved in the generation of conditioned neuronal responses in the rat frontal cortex.