A non-effective dose of exogenously applied L-dopa itself stereoselectively potentiates postsynaptic D2 receptor-mediated locomotor activities of rats. We further attempted to clarify whether or not endogenously released L-dopa tonically functions to potentiate activities of these receptors, simultaneously monitoring locomotor activities and basal release of L-dopa and dopamine during striatal microdialysis in conscious rats. Quinpirole (1 mg/kg, s.c.) alone, a selective D2 agonist, increased locomotor activities and decreased basal L-dopa and dopamine release 20-140 min after injection. Pretreatment with alpha-methyl-p-tyrosine (3 mg/kg, i.p.), a tyrosine hydroxylase inhibitor, decreased locomotor activities and further decreased L-dopa release without modification of dopamine release, compared to quinpirole alone, whereas 3-hydroxybenzylhydrazine (100 mg/kg, i.p.), a central dopa decarboxylase inhibitor, further increased locomotor activities and markedly increased L-dopa release without modification of dopamine release. Endogenously released L-dopa itself functions tonically to potentiate activities of postsynaptic D2 receptors relevant to locomotor movement of rats.