In rat hypothalamic slices, L-aromatic amino acid decarboxylase (AADC) was assayed, and the actions of L-DOPA on impulse (2 Hz)-evoked norepinephrine (NE) and dopamine (DA) release were studied under inhibition of AADC. Slices were incubated with L-DOPA, and DA and NE produced by conversion of the precursor were analyzed by high performance liquid chromatography with electrochemical detection (HPLC-ECD). In the slices, the Km and Vmax of AADC were 131 microM and 122 pmol/min/mg protein, respectively. NSD-1015, an AADC inhibitor, caused a noncompetitive type of inhibition, and the K1 value was 0.086 microM. In the presence of 20 microM NSD-1015, which was expected to cause 99.6% inhibition of AADC, L-DOPA (0.01-100 nM) concentration-dependently facilitated the release of NE from the superfused slices, and the L-DOPA (10 nM)-induced facilitation was antagonized by 100 nM ICI 89,406 and 100 nM ICI 118,551, a selective beta 1- and beta 2-adrenoceptor antagonist, respectively. This action of L-DOPA was not modified by 30 microM tropolone, an inhibitor of catechol-O-methyl-transferase. L-DOPA at 0.01-1 nM similarly facilitated the release of DA. A quantitative analysis revealed that the L-DOPA-induced increase in NE and DA release was much higher by a factor of 3 to 4 orders than was the amount of DA and NE converted from L-DOPA. These results add further support to the hypothesis that L-DOPA itself acts as a neuroactive substance in the rat central nervous system.