The effect exerted by some 1-alkoxybenzylidene-(4-toluyl-thiazol-2-yl)-hydrazines possessing monoamine oxidase inhibitory (MAOI) activity, on the thermotropic behaviour of model membranes constituted by dipalmitoylphosphatidylcholine (DPPC) vesicles, was studied by Differential Scanning Calorimetry (DSC). Attention was directed to evaluate eventual modifications in drug-lipid interaction induced by drug structural variations. The examined drugs, as hydrobromide compounds and their free bases, were found to modify the gel-to-liquid crystal phase transition of DPPC liposomes, by causing a shift of the transition temperature (Tm) towards lower values and a negligible variation in the enthalpy changes (Delta H). These modifications were a function of the drug concentration. These effects were differently modulated by the substituents present in the drug molecule backbone. Differences should be caused by the different polarity and pH influence of the salt or free base form. The calorimetric results could be related to MAO inhibitory activity measured by fluorescence techniques. The hypotheses on a relation between drugs structure, inhibitory activity and membrane interaction are suggested.