Accumulations of beta-amyloid protein are characteristic and diagnostic features of the brain of Alzheimer's disease patients; however, the physiological role of this protein in CNS is unknown. We have previously reported that continuous infusion of beta-amyloid protein into rat cerebral ventricle impairs learning ability and decreases choline acetyltransferase activity, a marker enzyme of cholinergic neuron. In this study, the effects of beta-amyloid protein infusion on the release of neurotransmitters in cholinergic and dopaminergic neuronal systems were investigated by using an in vivo brain microdialysis method. Nicotine-stimulated release of acetylcholine and dopamine in these animals was significantly lower than that in vehicle-infused rats. Further, dopamine release induced by high-K stimulation was decreased in beta-amyloid protein-infused rats compared with vehicle-infused rats. These results suggest that the release of the two transmitters, acetylcholine and dopamine, was decreased by beta-amyloid protein and that learning deficits observed in the beta-amyloid protein-infused rats are partly due to the impairment of neurotransmitter release. Furthermore, continuous infusion of beta-amyloid protein may be a useful method to produce the animal model of Alzheimer's disease.