The effects of eating bananas, a rich source of biogenic amines, on the plasma concentration of free and sulfate conjugated norepinephrine (NE) and dopamine (DA), and free epinephrine (E), were examined in normal male subjects before and after treatment with ascorbic acid, 2 g daily for 7 days. There were no significant changes in the levels of free NE or E in any subjects after eating a banana, either before or after ascorbic acid. Plasma free DA became detectable in some subjects, but the overall changes were not significant. Sulfate conjugated DA and NE increased markedly after banana ingestion, as previously demonstrated in our laboratory. After ascorbic acid treatment the rise in sulfate conjugated NE was attenuated, presumably because ascorbic acid acts as a competitive inhibitor of sulfate conjugation. In contrast, the rise in conjugated DA was potentiated after ascorbic acid treatment. This may be indicative of the higher affinity of DA for phenolsulfotransferase, an inhibitory effect of ascorbic acid on dopamine-receptor coupling or of ascorbic acid protecting DA from oxidation in the gut.