Trabajos originales Recibido el 22 de junio de 2007 Aceptado el 12 de agosto de 2007 SUMMARY. The effect of a liquid crystalline formulation in the in vitro antioxidant activity of quercetin was investigated using different methodologies in order to choose the most suitable method to perform the quality control and stability studies of the antioxidant activity of the liquid crystalline formulation added with quercetin. For all the assays, quercetin showed a dose-dependent activity having an IC 50 of 0.215 μg/mL in inhibiting lipid peroxidation, 1.125 μg/mL in inhibiting chemiluminescence produced in the H 2 O 2 /luminol/horseradish peroxidase (HRP) system and 0.834 μg/mL in DPPH • assay and the antioxidant activity of quercetin in liquid crystalline formulation was successfully evaluated in all the methodologies applied, once no interference by other components of the formulation was found and all the methodologies were sensitive for the quercetin evaluation. Thus, the liquid crystalline formulation containing vitamin E TPGS/ IPM / PG-H 2 0 (1:1) in the percentage of 63.75/21.25/15 (w/w/w), respectively, might be potential for cosmetic and pharmaceutical use, once it preserved the antioxidant effect of quercetin and all the methodologies applied could be used to perform the quality control and stability studies of the antioxidant activity of the liquid crystalline formulation added with quercetin. RESUMEN. "Influencia del Vehículo en la Actividad Antioxidante de la Quercetina: la Formulación Líquido Cristalina" quercetina. Para todos los métodos empleados, la quercetina presentó actividad dosis dependien-te con IC 50 de 0.215 μg/mL en la inhibición de la peroxidación lípida, 1.125 μg/mL en la inhibición de la qui-mioluminiscência generada por el sistema H 2 O 2 -HRP-luminol y 0.834 μg/mL en el ensayo de DPPH • y la activi-dad antioxidante de la quercetina adicionada a la formulación líquido cristalina fue valorada con suceso en todas las metodologías empleadas, ya que no hubo interferencia de los otros componentes da formulación y todas las metodologías se mostraron sensibles para evaluación de la quercetina. Así, la formulación líquido cristalina con-teniendo vitamina E TPGS/ MIP / PG-H 2 0 (1:1) en la porcentaje de 63.75/21.25/15 (p/p/p), respectivamente, po-de ser potente para administración cosmética o farmacéutica, una vez que preservó el efecto antioxidante de la quercetina y todas las metodologías empleadas pueden ser utilizadas en el control de calidad y estudios de estabi-lidad de la actividad antioxidante da formulación líquido cristalina contenido quercetina. INTRODUCTION Skin is a biological interface with the envi-ronment and functions as the first line of de-fense against noxious external stimuli such as ultraviolet, visible irradiation, prooxidant chemi-cals, infection and ionizing radiation 1 . However, by acute or chronic exposure to UV light the skin prooxidant/antioxidant equi-librium can be overwhelmed due to severe de-crease of its antioxidant content and to striking formation of reactive oxygen species (ROS). This way the ROS generated in excess can at-tack lipids in cell membranes, proteins in tissues or enzymes, carbohydrates and DNA. So the deleterious effects of sunlight and particularly UV radiation on the skin can lead to a variety of ravages as inflammation, skin aging, tumour promotion, cutaneous auto-immune disease, and phototoxicity/photosensitivity 1 . Topical administration of antioxidants pro-vides an efficient way to enrich the endoge-neous cutaneous protection system and thus may be a successful strategy for diminishing ul-traviolet radiation-mediated oxidative damage in