Rats with unilateral 6-hydroxydopamine lesions of the substantia nigra (SN) were challenged with L-DOPA (25 mg/kg, i.p.). One hour later, during the peak of rotational behavior, the animals were killed and the striatum and the SN were dissected and assayed for dopamine (DA) and dihydroxyphenylacetic acid (DOPAC) content. While L-DOPA treatment elevated DA levels in the lesioned striatum by only 10%, DA levels in the lesioned SN were completely restored to normal levels. DOPAC levels showed similar changes. In order to establish whether the large DA increase in the lesioned SN contributed to L-DOPA-induced contralateral circling, animals were implanted with chronic in-dwelling cannulas in the lesioned SN. Infusion of the DOPA decarboxylase inhibitor carbidopa (5 micrograms in 1 microliter) 30 min prior to peripheral L-DOPA injection not only reduced contralateral circling but reversed the direction of turning 20 min after the L-DOPA injection. The results are discussed in terms of dopaminergic regulation of the striatonigral pathway, their clinical relevance to Parkinson's disease and the suggestion that the SN is an important site for the action of L-DOPA.