Passive and iontophoretic transport of the model dipeptide tyrosine-phenylalanine (TyrPhe) that is subject to cutaneous metabolism and the uncharged glucose derivative benzyl-2-acetamido-2-deoxy-alpha-d-glucopyranoside (BAd-alpha-Glc) used as electroosmosis marker through heat-separated human epidermis was investigated in vitro. TyrPhe and BAd-alpha-Glc were used separately and in combination in order to determine their interaction in terms of permeability and the influence of skin metabolism of TyrPhe on permeation rate and tissue retention of itself and of BAd-alpha-Glc. TyrPhe was chemically and electrochemically stable but underwent considerable degradation in the epidermis under reflection boundary conditions with generation of degradation products tyrosine (Tyr) and phenylalanine (Phe) confirming cutaneous metabolism of TyrPhe in heat-separated human epidermis, which was more pronounced at pH 4.5 than at pH 3.0. As a result, no reproducible epidermis permeation of TyrPhe at pH 3 and no permeation at all at pH 4.5 was measured regardless of the presence of BAd-alpha-Glc, accompanied by increased levels of Tyr and Phe compared to blank runs. Low temperature (4 degrees C) at both pH values and addition of o-phenanthroline at pH 3 but not at pH 4.5 yielded reproducible TyrPhe permeation and blank, i.e., endogenous levels of Tyr and Phe evidencing inhibition of degradation. Constant voltage anodal iontophoresis marginally reduced BAd-alpha-Glc flux at pH 3 and 4.5 compared to the passive flux. In combination with TyrPhe, iontophoretic flux of BAd-alpha-Glc was increased markedly compared to the passive one when TyrPhe was metabolized in the tissue, while no such increase was observed when TyrPhe metabolism was inhibited. The increase of BAd-alpha-Glc iontophoretic flux was accompanied by a considerable decrease of the BAd-alpha-Glc amount retained in the epidermis. The presence of the generated Tyr and Phe, therefore, appears to be related to a decrease of the BAd-alpha-Glc amount retained in the epidermis upon application of an electrical voltage and an enhancement of its iontophoretic flux. Thus, an interaction between the concurrent permeants at the level of tissue retention induced by metabolism can influence the apparent iontophoretic permeation.