6-Hydroxydopamine-induced nerve terminal lesion of the nigrostriatal system may provide a partial lesion model of Parkinson's disease useful for the assessment of neuroprotective treatments and behavioral recovery after therapeutic intervention. The aim of the present study was to assess the retrograde degenerative changes in the dopaminergic neurons of the substantia nigra and the associated behavioral and neurochemical consequences of intrastriatal injections of 6-hydroxydopamine in young adult rats. Four groups of rats were stereotaxically injected in the right striatum with graded doses of 6-hydroxydopamine ranging from 0 to 20 mu g. Structural and functional deficits were quantified by tyrosine hydroxylase-immunoreactive nigral cell numbers, striatal dopamine content, skilled paw use, and drug-induced rotation. The results show that striatal 6-hydroxydopamine lesions produce dose-dependent decreases in striatal dopamine levels and tyrosine hydroxylase-immunoreactive cell numbers in the ipsilateral substantia nigra, accompanied by a significant long-lasting atrophy of the remaining dopaminergic neurons. Paw reaching test scores on the side contralateral to the lesion were non-linearly correlated with dopaminergic neuronal cell loss and exhibited a clear symptomatic threshold such that impaired paw use appeared only after >50% loss of nigral dopamine neurons or a reduction of 60-80% of striatal dopamine levels. The behavioral, cellular, and neurochemical effects of the nerve terminal lesion thus bear some resemblance to the early stages of Parkinson's disease, where the severity of motor impairment is correlated with the loss of dopamine in the striatum and dopaminergic neuronal loss in the substantia nigra. Rats with intrastriatal 6-hydroxydopamine lesions thus provide a model of progressive dopamine neuron degeneration useful not only for the exploration of neuroprotective therapeutic intervention but also for the study of mechanisms of functional and structural recovery after subtotal damage of the nigrostriatal dopamine system.