Most drugs have to pass cellular barriers in order to reach their site of action. This can be accomplished passively by diffusion, but more often it is an energy-consuming process using specific carrier proteins. Two groups of such proteins which are well characterised, the organic cation transporters and the multidrug resistance proteins, are discussed in detail in the present review. The clinical significance of these proteins is due not only to their role in drug distribution and elimination, but also to possible drug interactions when different drugs and/or endogenous substrates compete with the same carrier protein. Inhibition of multidrug resistance proteins could be of therapeutic value in impairing transport of drugs from their site of action and this could be particularly beneficial in the treatment of malignant diseases.