Small-angle neutron scattering (SANS) on the unilamellar vesicle (ULV) populations (diameter 500 and 1,000 A) in D2O was used to characterize lipid vesicles from dimyristoylphosphatidylcholine (DMPC) at three phases: gel Lbeta', ripple Pbeta' and liquid Lalpha. Parameters of vesicle populations and internal structure of the DMPC bilayer were characterized on the basis of the separated form factor (SFF) model. Vesicle shape changes from nearly spherical in the Lalpha phase to elliptical in the Pbeta' and Lbeta' phases. This is true for vesicles prepared via extrusion through pores with the diameter 500 A. Parameters of the internal bilayer structure (thickness of the membrane and the hydrophobic core, hydration and the surface area of the lipid molecule) were determined on the basis of the hydrophobic-hydrophilic (HH) approximation of neutron scattering length density across the bilayer rhox and of the step function (SF) approximation of rhox. DMPC membrane thickness in the Lalpha phase (T = 30 degrees C) demonstrates a dependence on the membrane curvature for extruded vesicles. Prepared via extrusion through 500 A diameter pores, vesicle population in the Lalpha phase has the following characteristics: average value of minor semi-axis 266 +/- 2 A, ellipse eccentricity 1.11 +/- 0.02, polydispersity 26%, thickness of the membrane 48.9 +/- 0.2 A and of the hydrophobic core 19.9 +/- 0.4 A, surface area 60.7 +/- 0.5 A2 and number of water molecules 12.8 +/- 0.3 per DMPC molecule. Vesicles prepared via extrusion through pores with the diameter 1,000 A have polydispersity of 48% and membrane thickness of 45.5 +/- 0.6 A in the Lalpha phase. SF approximation was used to describe the DMPC membrane structure in Lbeta' (T = 10 degrees C) and Pbeta' (T = 20 degrees C) phases. Extruded DMPC vesicles in D2O have membrane thickness of 49.6 +/- 0.5 A in the Lbeta' phase and 48.3 +/- 0.6 A in the Pbeta' phase. The dependence of the DMPC membrane thickness on temperature was restored from the SANS experiment.