To study behavioral character and changes of neuronal activity in the basal ganglia of ratmodel of levodopa-induced dyskinesia, unilateral 6-hydroxydopamine lesioned rat model of Parkin-son disease (PD) was treated with levodopa/benserazide twice daily for 4 weeks and the behaviorobserved on the 1st, 3rd, 4th, 7th, 9th, 10th, 14th, 21st and 28th day. The animals were sacri-ficed and immunohistochemical technique was used to measure the changes of Fos expression in thecaudate putamen (CPU), globus pallidus (GP) and sensorimotor area of cerebral cortex 2 h afterthe last treatment. The results showed that pulsatile treatment with a subthreshold dose of levodo-pa gradually induced abnormal involuntary movement (AIM), including stereotypy (limb dyskine-sia, axial dystonia and masticatory dyskinesia) towards the side contralateral to the dopamine-den-ervated striatum and increased contraversive rotation. The motor pattern of each subtype was highlystereotypic across individual rats, and the proportion of each subtype was not consistent among in-dividual rats. Fos positive nuclei in the CPU and GP were increased by levodopa acute administra-tion, and more remarkably in the CPU, but not in the cerebral cortex. After repeated levodopatreatment, Fos positive nuclei were reduced remarkably in the CPU, but were increased in the GPand cerebral cortex. It was concluded that the neural mechanisms underlying levodopa induced AIMin rat model of PD was very similar to those seen in levodopa-induced dyskinesia (LID) in PD pa-tients and MPTP-lesioned monkeys, and increased striatopallidal neuronal activity might be involvedin occurrence of LID.