Loss of dopaminergic neurons from the pars compacta of the substantia nigra is the pathological hallmark of Parkinson's disease (PD) and results in a partial deafferentation to the striatum. Since deafferentation is known to induce transynaptic atrophy of postsynaptic cells, we examined by Golgi impregnation the morphology of medium spiny I (MSI) striatal neurons, the principal target population for both nigrostriatal and corticostriatal fibers. Our quantitative data indicate that the dendritic arbor of MSI neurons in the putamen is significantly reduced in both length and number and MSI neurons are morphologically characterized by truncated dendrites with few dendritic spines and irregular, bulbous swellings. These data provide morphological evidence for the atrophy of striatal dendrites in PD and may explain, in part, the declining efficacy of chronic L-DOPA replacement therapy in advanced PD.