1. The present study has examined the influence of sodium chloride (0-160 mM) and ouabain (100 and 500 microM), an inhibitor of the enzyme Na(+)-K+ ATPase, on the synthesis of dopamine in slices of rat renal cortex loaded with exogenous L-dihydroxyphenylalanine (L-DOPA). The deamination of newly-formed dopamine into 3,4-dihydroxyphenylacetic acid (DOPAC) was also examined. The assay of L-DOPA, dopamine and DOPAC in kidney slices was performed by high performance liquid chromatography (h.p.l.c.) with electrochemical detection. 2. The accumulation of newly-formed dopamine and DOPAC in kidney slices loaded with L-DOPA (50 and 100 microM) was found to be dependent on the concentration of NaCl in the medium. A similar picture could be observed for DOPAC. The fractional rate of accumulation (k; mM NaCl-1) was at 50 and 100 microM L-DOPA, respectively, 0.00305 +/- 0.00036 and 0.00328 +/- 0.00029 for dopamine and 0.00672 +/- 0.00072 and 0.00641 +/- 0.00069 for DOPAC. The sodium-dependent formation of dopamine was completely abolished when the experiments were performed in the absence of oxygen. 3. In experiments performed in the presence of 120 mM NaCl, but not in conditions of low sodium (20 mM NaCl in the medium), ouabain (100 and 500 microM) was found to inhibit the accumulation of newly-formed dopamine and DOPAC (14-57% reduction; P less than 0.05); this effect was more marked at 50 and 100 microM L-DOPA. When the experiments were performed in the absence of oxygen the renal production of dopamine and DOPAC became markedly decreased (respectively, 40% and 77% reduction; P<0.05) and ouabain (100 and 500 microM) was no longer able to reduce the accumulation of both newly-formed dopamine and DOPAC.4. The formation of dopamine and its deamination to DOPAC in kidney homogenates closely depended on the concentration of L-DOPA added to the medium; ouabain (100, 500 and 1000 microM) was found to affect neither the formation of dopamine nor its deamination to DOPAC when homogenates were used instead of kidney slices.5. In conclusion, the results presented here show that the formation of dopamine in kidney slices loaded with L-DOPA is sodium-dependent and ouabain-sensitive suggesting the involvement of a co-transport system of sodium and DOPA into the tubular epithelial cell.