This study examined whether dopamine (DA) is necessary for the normal development of striatal enkephalin and striatonigral tachykinin peptide systems. The neurotoxin, 6-hydroxydopamine (6-OHDA) was used to induce DA deficiency on the third day of the postnatal period in Sprague-Dawley rat pups. The animals were sacrificed at 60 days of age. The levels of Met5-enkephalin (ME) and substance P (SP) were determined by radioimmunoassay and preproenkephalin (PPE) and preprotachykinin (PPT) mRNA abundance in the striatum were assessed by hybridization analysis. The concentrations of DA, 5-hydroxytryptamine (5-HT) and their acid metabolites were determined by high-pressure liquid chromatography with electrochemical detection. The lesioned animals were grouped on the basis of the degree of loss of DA, and changes in ME, SP and 5-HT systems were correlated with respect to the degree of DA loss. The nature and extent of the changes in these systems were dependent on the degree of DA depletion. A loss of more than 90% DA was necessary to result in increased levels of ME and its PPE mRNA and reduced levels of SP and its PPT mRNAs; however, increased levels of 5-HT could be observed at a lower degree of DA loss. The results indicate that the normal development of enkephalin and tachykinin and 5-HT systems of basal ganglia are dependent on the availability of DA and/or the integrity of the nigrostriatal dopaminergic neurons. The results are relevant to our further understanding of the neurobiology of DA deficiency disorders.