The temperature dependence of intermembrane interactions in freely suspended multilamellar membranes of dimiristoylphosphatidylcholine in D2O was studied using small-angle neutron scattering (SANS) and high-resolution x-ray diffraction (HRXRD). The study reveals that the Helfrich's undulation force is the dominating repulsion force at temperatures above 48.6 degrees C and intermembrane distances larger than 20.5 A. At approximately 77 degrees C the onset of the unbinding transition in the multilamellar membranes is observed. This transition has a continuous behavior in agreement with theoretical predictions and proceeds in accordance with a two-state model. Complimentary analysis of SANS and HRXRD data permits accurate calculation of the fundamental undulation force constant cfl. The obtained value of cfl=0.111+/-0.005 is in good agreement with theoretical calculations. The results of this work demonstrate a key role of Helfrich's undulations in the balance of intermembrane interactions of lipid membranes under physiological temperatures and suggest that thermal undulations play an important part in the interactions of biological membranes. The agreement of the predictions with the experimental data confirms that lipid membranes can be considered as random fluctuating surfaces that can be described well by current theoretical models and that they can serve as a powerful tool for studying behavior of such surfaces.