Using a microdialysis technique, the rat striatum was perfused with NSD-1015, an inhibitor of aromatic L-amino acid decarboxylase, and the amount of L-3,4-dihydroxyphenylalanine (L-DOPA) and 5-hydroxytryptophan (5-HTP) accumulating in dialysate was measured as an index of in vivo activities of tyrosine hydroxylase and tryptophan hydroxylase.