The effect of soluble aspirin on the availability of vitamin C has been studied in guinea-pigs and human subjects. In the human study, the concentrations of vitamin C in plasma, leucocytes and urine were found to be markedly elevated at various intervals following administration of a single oral dose of 500 mg of the vitamin. The vitamin C-associated increases, however, appeared to be blocked when the vitamin was given simultaneously with aspirin (900 mg). Similar findings were observed in guinea-pigs, where in addition faecal excretion of vitamin C was found to be significantly increased when the vitamin was administered together with aspirin. These results suggest that aspirin may impede gastrointestinal absorption of vitamin C. This hypothesis has been strengthened with in vitro studies using everted gut sac preparations where both the serosal/mucosal concentration gradient and the uptake of vitamin C per unit weight of intestine were markedly lowered by acetyl-salicylate. Such an interaction is relevant to the population where vitamin C intake is borderline.