Imipramine penetration into the lipid core of a membrane was demonstrated through measurements on lipid monolayers (surface pressure and surface potential). The surface pressure measurements allow us to calculate the intrinsic binding constant (partition coefficient) for the lipid-Imipramine interaction. This latter value is in correct agreement with the results obtained by electrophoretic mobility measurements on liposomes. In addition, it was observed that the same mole fraction of "lipid-soluble drug" (Chlorpromazine or Imipramine) incorporated in a given lipidic phase (DPPC) induced the same shift in the transition temperature.