The influence of Gramicidin D (GD) incorporation on the structure and phase behavior of aqueous dispersions of DMPC lipid bilayers has been studied using small-angle x-ray scattering (SAXS) and (2)H-NMR spectroscopy. The experiments covered a temperature range from -10 degrees C to 60 degrees C and a pressure range of 0.001-4 kbar. Pressure was used to be able to tune the lipid bilayer conformational order and phase state and because high pressure is an important feature of certain natural biotopes. The data show that, depending on the GD concentration, the structure of the temperature- and pressure-dependent lipid phases is significantly altered by the insertion of the polypeptide, and a p,T-phase diagram could be obtained for intermediate GD concentrations. Upon gramicidin insertion, a rather narrow fluid-gel coexistence regions is formed. Two gel phases are induced which are different from those of the pure lipid bilayer system and which separate at low temperatures/high pressures. For both the temperature- and pressure-induced fluid-to-gel transition, a similar pseudocritical transitional behavior is observed, which is even more pronounced upon incorporation of the peptide.