The interactions between synthetic lecithins (DMPC and DPPC) and meperidine, methadone and naloxone were determined by means of differential scanning calorimetry and monomolecular layers. The results of the calorimetric measurements show that only for the most hydrophobic molecules do hydrophobic interactions have a significant value. Naloxone and meperidine interact mainly electrostatically with the polar head groups of DMPC and DPPC. Similar behaviour was observed when studying the compression isotherms of PC lipid monolayers in the presence of these opiates and the penetration kinetics of the same molecules in monolayers. Moreover the differently ordered states of the molecules in monolayers, gel phase (DPPC) or liquid crystalline phase (PC) at room temperature greatly influences their interaction with opiate molecules.