The synthesis of dopamine (DA) and the extent of its deamination to dihydroxyphenylacetic acid (DOPAC) in rat kidney slices loaded with L-DOPA, under in vitro conditions, has been studied. Inhibition of MAO-A increased the accumulation of newly-formed DA and reduced DOPAC formation; by contrast, the inhibition of monoamine oxidase, MAO-B, only resulted in a decrease in DOPAC formation. Raising the sodium chloride concentration in the incubation medium increases DA formation and reduces DOPAC accumulation. The addition of α-human atrial natriuretic peptide (α-hANP; 3.3 and 330 nmol/L) to the incubation medium decreases DA formation; this effect was potentiated by the cGMP phosphodiesterase inhibitor M&B 22,948 and antagonized by a raise in the sodium chloride concentration to 157 mmol/L. Am J Hypertens 1990;3:7S-10S