  We use the tapering effect of Halpha/[NII] rotation curves of spiral galaxies
first noted by Goad & Roberts (1981) to investigate the internal extinction in
disks. The scale length of exponential fits to the inner part of rotation
curves depends strongly on the disk axial ratio. Preliminary modelling of the
effect implies substantial opacity of the central parts of disks at a
wavelength of 0.66 \mu. In addition, the average kinematic scale length of
rotation curves, when corrected to face--on perspective, has a nearly constant
value of about 1.7 h^{-1} kpc, for all luminosity classes. The interpretation
of that effect, as the result of the increasing dominance of the baryonic mass
in the inner parts of galaxies, yields a mean baryonic mass--to--light ratio in
the I band \Upsilon_I= 2.7h M_sun/L_{sun,I}, within the inner 1.7 h^{-1} kpc of
disks.
