  We derive radial profiles of the surface-mass-density for 19 spiral galaxies
directly from their high-resolution rotation curves. Using the corresponding
luminosity profiles, we obtain the radial distribution of the
mass-to-luminosity ratios ($M/L$) from the inner bulge ($\sim$ a few 100 pc) to
the outer disk ($\geq$ 2-10 kpc) for 11 galaxies (with inclination <
$70^{\circ}$ in order to reduce the influence of the interstellar extinction.
The $M/L$s in the bulges of two galaxies with sufficient resolution, NGC 4527
and NGC 6946, are found to increase steeply toward the center at radii $\sim$
100-500 pc at rates of 15$\pm$3 and 7$\pm$2 times per kpc, respectively. Some
other galaxies with fairly high resolution also show signs of an increase
toward the center. Such an increase may indicate the existence of a new
component, a ``massive dark core'', which may be an object linking the bulge
and a central black hole. Based on radial variations of the $M/L$, we further
discuss the variation of the dark-mass fraction in spiral galaxies.

Astrophysics