Xanthine oxidase is a flavoprotein enzyme which catalyzes the oxidative hydroxylation of purine substrates. Because of its availability, it has become a model for structural molybdoenzymes in general. The enzyme is a well-established target of drugs against gout and hyperuricemia and exists in two forms: oxidase and deshydrogenase. In sonic pathologies, its level increases in oxidase form, being the source of free radicals which can cause damage to surrounding tissues. It is important to understand the mechanisms of the enzyme inhibition to help in the search of new inhibitors. The main active center is a molybdopterin buried in a cavity. Theoretical calculations can be of some help for distinguishing the important aspects in the inhibition: attraction inside the cavity and anchorage. In this paper, the molybdopterin molecule geometry has been optimized by ab initio, with the DFT method and the results have been shown to be very similar to the X-ray coordinates. In order to evaluate the attraction inside the cavity, the electrostatic potential between the charged molybdopterin molecule and two series of inhibitors, some flavonoids, and sonic gallic acid derivatives have been calculated using the multipolar development Supplied by the Gaussian package. The good concordance between the electrostatic force and IC50 indicates that the attraction is an important factor in the inhibition and must be. taken into account in the designing of new drugs.