Dopamine (DA) release and metabolism within the substantia nigra (SN) were studied in normal rats, rats with unilateral 6-hydroxydopamine (6-OHDA) lesions, and 6-OHDA-lesioned rats treated with glial cells line-derived neurotrophic factor (GDNF). Animals with > 99% DA depletions, as determined by apomorphine-induced circling behavior, also showed significant deficits in several measures of spontaneous motor activity. In vivo microdialysis recordings in the SN were carried out in normal and unilaterally 6-OHDA-lesioned rats. Basal levels of DNA were detectable only in the dialysates of normal animals, and basal levels of t he primary DA metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid were found to be significantly reduced in the SN of 6-OHDA-lesioned animals. In the presence of d-amphetamine, either alone or in combination with potassium, significant reductions in DA release were observed in the SN of 6-OHDA-lesioned animals compared to normal animals. Potassium-evoked DA release alone was not significantly different between the groups. A single intranigral administration of GDNF into 6-OHDA-lesioned animals elicited a significant reduction in apomorphine-induced rotation behavior and a significant increase in spontaneous motor activities. These behavioral changes were apparent at 1 week and persisted through 4 weeks following treatment. In vivo microdialysis showed that, although DA metabolism was altered 1 week following GDNF treatment, DA release was not significantly affected until 4 weeks following treatment.