Acetylcholine was collected from the somatosensory cortex of anesthetized rats, using the microdialysis technique. Electrical stimulation of the nucleus basalis magnocellularis (NBM) with trains of 10 pulses at 100 Hz delivered every second produced a 3-4-fold increase in acetylcholine release. Stimulation with an intratrain frequency of 10, 50, 100 or 200 Hz demonstrated that 100 Hz trains produced the greatest increase, while the other frequencies were about half as effective. The cortical release of acetylcholine in this paradigm supports the hypothesis that the previously demonstrated enhancement by NBM stimulation of cortical sensory inputs is due to cholinergic activation.