Rats received a unilateral lesion of the nucleus basalis magnocellularis (NBM) by infusion of ibotenic acid. In addition, the dorsal raphe nucleus was lesioned by infusion of 5,7-dihydroxytryptamine (5,7-DHT). The release of acetylcholine (ACh), choline, serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) was measured in the frontal neocortex by means of microdialysis. Lesions of the NBM, but not the raphe nucleus, reduced the release of ACh significantly (-47%). The release of 5-HT and 5-HIAA was reduced by raphe lesions (-44% and -79%+), but not by NBM lesions. In no case did the combined lesion affect neurotransmitter release more than a single lesion. These results suggest that serotonergic projections from the dorsal raphe nucleus are not involved in tonic inhibition of ACh release in the neocortex.