The effect of focal stimulation of the magnocellular nucleus of the basal forebrain (nucleus basalis of Meynert; NBM) on acetylcholine (ACh) release in the cerebral cortex in the parietal lobe was examined in halothane-anesthetized rats. ACh was measured using a microdialysis method. Focal electrical stimulation of the unilateral NBM increased ACh release in the ipsilateral cerebral cortex in stimulus intensity and frequency dependently. Microinjection of L-glutamate (100 nmol) into the unilateral NBM also increased ACh release in the ipsilateral cerebral cortex. The ACh release in the contralateral cerebral cortex was not affected by these unilateral stimulations of the NBM. It was concluded that focal stimulation of the NBM releases ACh from cortical terminals of cholinergic fibers originating in the NBM.