  We propose that the lower bound of the stellar radial velocity dispersion c_r
of an equilibrium stellar disk is determined by the gravitational stability
condition. We compared the estimates of stellar velocity dispersion at radii
r>(1.5 - 2)R_0 (where R_0 is the photometric radial scalelength of a disk),
found in the literature, with the minimal values of c_r necessary for the disk
to be in a stable state, using the results of numerical simulations of 3D
collisional disks. This approach enables to estimate an upper limit of the
local surface density and (if R_0 is known) a total masses of a disk and a dark
halo. We argue that the old stellar disks of spiral galaxies with active star
formation usually have the velocity dispersions which are close to the expected
marginal values. A rough values of disk-to-total mass ratios (within the fixed
radius) are found for about twenty spiral galaxies. Unlike spirals, the disks
of the ``red'' Sa--S0 galaxies are evidently ``overheated'': their radial
dispersion of velocities at r ~ 2 R_0 exceeds significantly the marginal values
for gravitational stability.
