The method for non-invasively determining the relative levels of two substances present in a biological system includes the steps of extracting by reverse iontophoresis charged and uncharged substances from said biological system, and collecting said substances, analysing the collected amount of a first extracted substance and a second extracted substance and determining the extraction ratio of the first substance to the second substance to determine their relative levels in the biological system. When the first and second substances are both analytes susceptible to changes in their concentration in the biological system, this method provides information about the relative concentrations of the two substances in the biological system. When the first substance is an analyte susceptible to changes in its concentration in the biological system and the second substance has a substantially constant concentration in the biological system, the second subtance acts as an internal standard and the extraction ratio becomes a direct measurement of the physiological level of the first substance in the biological system. A iontophoretic sampling device for monitoring the relative levels of two substances present in a biological system is also disclosed.Procédé servant à déterminer de façon non invasive les niveaux relatifs de deux substances présentent dans un système biologique et consistant à extraire dudit système biologique par iontophorèse inverse des substances chargées et non chargées, à recueillir lesdites substances, à analyser la quantité recueillie dune première substance extraite et dune deuxième substance extraite et à déterminer le rapport dextraction entre la première substance et la deuxième substance afin de définir leur niveau relatif dans le système biologique. Quand cette première et cette deuxième substance consiste en des substances à analyser susceptibles de subir des modifications de concentration dans le système biologique, ce procédé permet dobtenir des
