The effect of a unilateral 6-hydroxydopamine (6-OHDA) lesion of the medial forebrain bundle or of a sham lesion on the neuropeptide content of the striatum and substantia nigra was investigated with or without 6 months L-3,4-dihydroxyphenylalanine (L-DOPA; 200 mg/kg per day) plus carbidopa (25 mg/kg per day) treatment. [Met5]- and [Leu5]enkephalin, substance P (SP), neurotensin (NT) and cholecystokinin (CCK) were measured by a combined HPLC/RIA method. Neurotensin levels were increased in the striatum, and [Leu5]enkephalin, and SP levels were reduced in the substantia nigra as a consequence of the lesion, while the levels of other peptides were unaltered. Administration of L-DOPA to sham-operated rats bilaterally increased SP levels in striatum and substantia nigra, and [Met5]enkephalin and CCK content in substantia nigra. L-DOPA treatment of 6-OHDA-lesioned rats increased [Met5]- and [Leu5]enkephalin and CCK levels in the striatum ipsilateral to the lesion but not on the intact side. In the substantia nigra, the lesion-induced decrease in [Leu5]enkephalin and SP was reversed by L-DOPA treatment, [Met5]enkephalin and CCK levels ipsilateral to the lesion were further enhanced, and there was an increase in NT ipsilateral to the lesion. Cryptic [Met5]- and [Leu5]enkephalin increased in the ipsilateral striatum following an 6-OHDA lesion. L-DOPA treatment did not alter cryptic enkephalin levels or the lesion-induced increase in cryptic [Met5]enkephalin, while cryptic [Leu5]enkephalin was further increased in lesioned animals given L-DOPA. These results suggest that the pattern of change in basal ganglia peptides in Parkinson's disease is not due solely to the destruction of the nigrostriatal pathway, the drug treatment of the disease or a combination of these factors.