  We have studied the mesoscopic shape fluctuations in aligned multilamellar
stacks of DMPC bilayers using the neutron spin-echo technique. The
corresponding in plane dispersion relation $\tau^{-1}$(q$_{||}$) at different
temperatures in the gel (ripple, P$_{\beta'}$) and the fluid (L$_{\alpha}$)
phase of this model system has been determined. Two relaxation processes, one
at about 10ns and a second, slower process at about 100ns can be quantified.
The dispersion relation in the fluid phase is fitted to a smectic hydrodynamic
theory, with a correction for finite q$_z$ resolution. We extract values for,
the bilayer bending rigidity $\kappa$, the compressional modulus of the stacks
$B$, and the effective sliding viscosity $\eta_3$. The softening of a mode
which can be associated with the formation of the ripple structure is observed
close to the main phase transition.
