Rotenone, which is used as a pesticide and insecticide, has been shown to cause systemic inhibition of mitochondrial complex I activity, with consequent degeneration of dopaminergic neurons within the substantia nigra and striatum, as observed in Parkinson's disease. A novel intrastriatal rotenone model of Parkinson's disease was used to examine the neuroprotective effects of valproic acid (VPA), which is known to upregulate neurotrophic factors and other protective proteins in the brain. Sham or lesioned rats were treated with either vehicle or VPA at a dose of 4mg/mL in drinking water. The right striatum was lesioned by infusion of rotenone at three sites (2μg/site) along its rostro-caudal axis. A forelimb asymmetry (cylinder) test indicated a significant (p<0.01) decrease in use of the contralateral forelimb in rotenone-lesioned animals, in the third week post-lesioning, which was abolished by VPA treatment. Similarly, a significant (p<0.01) and persistent increase in use of the ipsilateral forelimb in lesioned animals over the 4weeks of testing, was not seen in animals treated with VPA. Results of the asymmetry test illustrate that intrastriatal infusion of rotenone causes contralateral motor dysfunction, which is blocked by VPA. The significant increase in ipsilateral forelimb use has not been documented previously, and presumably represents a compensatory response in lesioned animals. Six weeks post-surgery, animals were sacrificed by transcardial perfusion. Subsequent immunohistochemical examination revealed a decrease in tyrosine hydroxylase immunoreactivity within the striatum and substantia nigra of rotenone-lesioned animals. VPA treatment attenuated the decrease in tyrosine hydroxylase in the striatum and abolished it in the substantia nigra. Stereological cell counting indicated a significant (p<0.05) decrease in tyrosine hydroxylase-positive dopamine neurons in the substantia nigra of rotenone-lesioned animals, which was confirmed by Nissl staining. Importantly, this loss of dopamine neurons in rotenone-lesioned animals, was blocked by chronic VPA treatment. These findings strongly support the therapeutic potential of VPA in Parkinson's disease.