Hydroxytyrosol (HT), one of the major polyphenols present in olive oil, is known to possess a high antioxidant capacity. The aim of the present study was to investigate dose dependent (0, 1, 10 and 100 mg/kg) alterations in the metabolism of HT in rats since it has been reported that metabolites may contribute to biological effects. Special attention was paid to the activation of the semiquinone-quinone oxidative cycle and the formation of adducts with potential deleterious effects. Thus, we developed a novel analytical methodology to monitor the in vivo formation of the HT mercapturate, N-acetyl-5-S- cysteinyl-hydroxytyrosol in urine samples. Biomarkers of hepatic and renal toxicity were evaluated within the dose range tested. Following HT administration, dose-dependent effects were observed for the recovery of all the metabolites studied. At the lowest dose of 1 mg/kg, the glucuronidation pathway was the most relevant (25-30%), with lower recoveries for sulfation (14%), while at the highest dose of 100 mg/kg, sulfation was the most prevalent (75%). In addition, we report for the first time the formation of the mercapturate conjugate of HT in a dose-dependent manner. The biochemical data did not reveal significant toxic effects of HT at any of the doses studied. An increase in the GSH/GSSG ratio at the highest dose was observed indicating that the products of HT autoxidation are counteracted by glutathione, resulting in their detoxification. These results indicate that the metabolic disposition of HT is highly dependent on the dose ingested.