Résumé : The conformation of three imidazole derivatives, miconazole, ketoconazole and deacylated ketoconazole (R 39 519) inserted in a lipid layer was calculated using a procedure of conformational analysis. For each imidazole derivative all probable conformers were inserted into a dipalmitoyl phosphatidylcholine (DPPC) monolayer. Miconazole maintains its two dichlorophenyl groups in the hydrophobic phase whereas the imidazole moiety is orientated in the hydrophilic phase. Ketoconazole orientates its dichlorophenyl group in the hydrophobic phase whereas its acylated piperazine moiety is orientated towards the hydrophobic region. Deacylation inverses completely the orientation of the compound. The most probable conformer of R 39 519 is inserted in the lipid layer with its piperazine moiety orientated towards the aqueous phase. The inversion increases the area occupied per drug molecule from 30 A2 for ketoconazole to 90 A2 for R 39 519 equal to the mean area occupied per miconazole molecule and higher than that occupied per DPPC molecule (60 A2). Such a conformation should result in a destabilizing effect of miconazole and R 39 519; this was proved using differential scanning calorimetry.