Zinc is able to form complexes with 8-hydroxyquinoline (HC). Therefore the penetration of HC from topical formulations into the ear of guinea-pigs and into a multilayer membrane system was decreased by Zi. Using the AUC a suitable in vitro-in vivo correlation was obtained. An exception was observed when there were interactions between the ointment base and the in vitro model system. Furthermore, it was found that also zincoxinate is able to penetrate into the skin of guinea-pigs.