1,3-dipalmitoyl-2-(4-aminobutyryl)glycerol • HCI, a diglyceride ester of 7-aminobutyric acid (GABA), was synthesized with a good yield (46.5%). This prodrug ( 1,3-dp-GABA " HCI) was highly amphiphilic and formed stable dispersions in water. The thermodynamic parameters of the endothermic transition were determined by differential scanning calorimetry, The lipid behaviour was strongly dependent on the thermal history of the sample: a pretransition was only observed with sonicated dispersions on the first heating scan. X-ray diffraction indicated a crystalline or paracrystalline organization below the transition temperature and a hexagonal or lamellar organization above this temperature; furthermore, it revealed a very rich wide angle pattern even in the presence of a large amount of water. Freeze fracture electron microscopy was used to investigate the morphology of the aqueous dispersions: large unilamellar vesicles were observed at all temperatures with unsonicated dispersions. Sonicated dispersions revealed fairly unusual behaviour-a reversible equilibrium was obtained between a lamella-or disk-like structure and a vesicular structure, below and above the transition temperature, respectively.