Hydroxytyrosol (HOTYR) and tyrosol (TYR), main phenolic compounds of olive oil, have been reported to contribute to the prevention of cardiovascular diseases due to their antioxidant activities, e.g., protection of low-density lipoprotein (LDL) oxidation. Their bioavailability in humans is poor, and they are found in biological fluids mainly as conjugated metabolites. Low concentrations of free phenols are unlikely to explain the biological activities seen in humans after olive oil intake. In this context, antioxidant activities of conjugated metabolites, in a range of concentrations compatible with their dietary consumption, were evaluated. Concentrations of metabolites and their core compounds were estimated in an intervention study of 11 healthy volunteers whose diet was supplemented with 50 ml of virgin olive oil, using high-performance liquid chromatography coupled to mass spectrometry for the simultaneous analysis of 3'-O- and 4'-O-HOTYR-glucuronides and 4'-O-glucuronides of TYR and homovanillyl alcohol in human urine. Glucuronides and core compounds were tested for their chemical (hydrogen donation by 1,1-diphenyl-2-picrylhydrazyl free radical test) and in vitro biological (inhibition of Cu-mediated LDL oxidation) antioxidant activities at the concentration ranges observed in human biological fluids (range, 0.01-10 muM) after dietary olive oil consumption. None of the glucuronides displayed significant antioxidant activities at the concentrations tested.