Extraneural dopamine is thought to be synthesized by an aromatic L-amino acid decarboxylase (L-AADC) activity in tubular cells. However, the previous histochemical studies of this enzyme's localization in the nephron were not consistent. To determine the localization of L-AADC and whether changes in Na intake regulate this enzyme, L-AADC was measured in microdissected nephron segments from rat kidneys. Dopamine formed by isolated tubules incubated with exogenous L-dopa was quantitated by high-performance liquid chromatography (HPLC) and with the more sensitive radioenzyme assay (REA). L-AADC activity was present only in proximal convoluted (PCT, 208 +/- 19 ng.cm-1.h-1) and proximal straight tubules (PST, 81 +/- 9 ng.cm-1.h-1), whereas no significant activity was detected in other nephron segments by either HPLC or REA. Maximal velocity (Vmax) of L-AADC in a low-salt diet group (246 +/- 4 ng.cm-1.h-1) showed a small but a significant decrease (P less than 0.05) compared with control and high-salt diet groups (311 +/- 6 and 293 +/- 4 ng.cm-1.h-1, respectively), whereas the apparent Michaelis constant (Km) was similar in these three groups. These results show that L-AADC is present only in the PCT and PST of the rat nephron, and suggest that the changes in L-AADC activity may contribute to the regulation of extraneural dopamine production in the kidney during low-salt intake.