Changes in the plasma concentration of free and conjugated noradrenaline (NA) and dopamine (DA) following oral administration of these catecholamines were examined in four normal males before and after treatment with ascorbic acid.The marked increase in plasma conijugated DA and NA was considerably reduced after ascorbic acid treatment, presumably due to competitive inhibition of sulphate conjugation. In one subject where there was a considerable delay in the appearance of sulphates, an early rise in free NA and DA concentration occurred. There were no changes in plasma free NA in the other subjects, although DA increased in two after ascorbic acid treatment.These findings emphasise the importance of sulphate conjugation in the gut or liver in inactivating endogenous amines in foods and indicate that interference of this protective mechanism may arise from high doses of ascorbic acid and other related compounds which compete for sulphation.