Ferrocenyl dendrimers based on octasilsesquioxane cores electrodeposited on the electrode surface form a 3D structured film with an interesting highly effective surface. This property, together with the presence of interacting ferrocenes, make these films useful electrode materials for two-electron exchanges. This work presents the morphologic and kinetic studies of electrodes modified with a second-generation octasilsesquioxane dendrimer with 32 ferrocenyl terminal groups and their application in the determination of L-tryptophan (TR). The modified electrode shows a reversible redox system without kinetic limitations and allows to determine TR in two wide linear ranges (from 0.06 to 100 mu M and from 100 to 440 mu M), with low detection limit (0.01 mu M). In addition, the modified electrode is successfully applied to detect TR in presence of ascorbic acid, dopamine and uric acid and to their simultaneous determination. The new device has been also applied to the direct determination of TR in pharmaceutical and biological samples with satisfactory results.