Several 2- or 8-n-alkyl-hypoxanthines and a 2,8-di-n-pentyl-hypoxanthine were synthesized and tested as substrates or inhibitors of Xanthine Oxidase (XO). 8-Alkyl derivatives showed a substrate behaviour, whereas 2-alkyl substituted compounds were non-substrates and inhibitors. 2,8-di-n-pentyl-hypoxanthine was ineffective as inhibitor. The comparison between their activity allowed us to conclude that the complexes formed by the enzyme and the cited n-alkylhypoxanthines or 2-n-alkyl-8-azahypoxanthines involve their N(3) and N(9) positions in all the cases. The position of the n-alkyl chain determines the disposition of the molecule inside the complex: 2-n-alkyl-hypoxanthines and 2-n-alkyl-8-azahypoxanthines gave complexes with the same orientation of heterocyclic moieties, opposite that given by 8-n-alkyl-hypoxanthines.