We have used intracerebral dialysis to monitor the striatal extracellular fluid (ECF) in rats with unilateral lesions of the nigrostriatal dopamine (DA) pathway. Dialysis samples were collected before and after L-dihydroxy-phenylalanine (L-DOPA) administration both in the presence and absence of carbidopa, an extracerebral DOPA decarboxylase (DDC) inhibitor. The baseline ECF levels of DA, 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) were always higher in the intact than in the lesioned striata. In the normal striata, dopamine (DA) concentrations increased following L-DOPA administration. Pretreatment with carbidopa prolonged the duration of the DA increase. In the lesioned striata, DA levels increased following L-DOPA administration only in animals pretreated with carbidopa. Following L-DOPA administration, striatal HVA and DOPAC levels increased considerably more in animals not pretreated with carbidopa than they did in pretreated animals. This increase was particularly marked in the lesioned striata and leads us to conclude that extracerebrally produced HVA and DOPAC can enter the brain extracellular space.